Method for supporting handover and corresponding apparatus

ABSTRACT

The present disclosure relates to a communication method and system for converging a 5 th -Generation (5G) communication system for supporting higher data rates beyond a 4 th -Generation (4G) system with a technology for Internet of Things (IoT). The present disclosure may be applied to intelligent services based on the 5G communication technology and the IoT-related technology, such as smart home, smart building, smart city, smart car, connected car, health care, digital education, smart retail, security and safety services. 
     The present invention provides a method of a base station for supporting an inter-system handover from an evolved packet system (EPS) system to a 5th generation (5G) system. The method comprises receiving, from an access and mobility management function (AMF), a handover request message including E-UTRAN radio access bearer (E-RAB) information and quality of service (Qos) flow information, identifying whether a protocol data unit (PDU) session is accepted or not, determining whether a data forwarding for at least one Qos flow associated with the PDU session is accepted or not based on the E-RAB information and the Qos flow information in case that the PDU session is accepted and transmitting, to the AMF, a handover request acknowledgement message including information on the at least one Qos flow based on the determination. The method solves the data forwarding problem during the movement of a UE between an LTE system and a 5G system, so that the loss of data is avoided and the continuity of services is ensured.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 16/638,349,now U.S. Pat. No. 11,160,002, which is the 371 National Stage ofInternational Application No. PCT/KR2018/009248, filed Aug. 13, 2018,which claims priority to Chinese Patent Application No. 201710682812.2,filed Aug. 11, 2017, Chinese Patent Application No. 201710944124.9,filed Sep. 30, 2017, and Chinese Patent Application No. 201711194424.6,filed Nov. 24, 2017, the disclosures of which are herein incorporated byreference in their entirety.

BACKGROUND 1. Field

The present invention relates to the technical field of radiocommunication, and in particular to a method for supporting handover anda corresponding apparatus.

2. Description of Related Art

To meet the demand for wireless data traffic having increased sincedeployment of 4G communication systems, efforts have been made todevelop an improved 5G or pre-5G communication system. Therefore, the 5Gor pre-5G communication system is also called a ‘Beyond 4G Network’ or a‘Post LTE System’. The 5G communication system is considered to beimplemented in higher frequency (mmWave) bands, e.g., 60 GHz bands, soas to accomplish higher data rates. To decrease propagation loss of theradio waves and increase the transmission distance, the beamforming,massive multiple-input multiple-output (MIMO), Full Dimensional MIMO(FD-MIMO), array antenna, an analog beam forming, large scale antennatechniques are discussed in 5G communication systems. In addition, in 5Gcommunication systems, development for system network improvement isunder way based on advanced small cells, cloud Radio Access Networks(RANs), ultra-dense networks, device-to-device (D2D) communication,wireless backhaul, moving network, cooperative communication,Coordinated Multi-Points (CoMP), reception-end interference cancellationand the like. In the 5G system, Hybrid FSK and QAM Modulation (FQAM) andsliding window superposition coding (SWSC) as an advanced codingmodulation (ACM), and filter bank multi carrier (FBMC), non-orthogonalmultiple access (NOMA), and sparse code multiple access (SCMA) as anadvanced access technology have been developed.

The Internet, which is a human centered connectivity network wherehumans generate and consume information, is now evolving to the Internetof Things (IoT) where distributed entities, such as things, exchange andprocess information without human intervention. The Internet ofEverything (IoE), which is a combination of the IoT technology and theBig Data processing technology through connection with a cloud server,has emerged. As technology elements, such as “sensing technology”“communication and network infrastructure” “service interfacetechnology” “Security technology” have been demanded for IoTimplementation, a sensor network, a Machine-to-Machine (M2M)communication, Machine Type Communication (MTC), and so forth have beenrecently researched. Such an IoT environment may provide intelligentInternet technology services that create a new value to human life bycollecting and analyzing data generated among connected things. IoT maybe applied to a variety of fields including smart home, smart building,smart city, smart car or connected cars, smart grid, health care, smartappliances and advanced medical services through convergence andcombination between existing Information Technology (IT) and variousindustrial applications.

In line with this, various attempts have been made to apply 5Gcommunication systems to IoT networks. For example, technologies such asa sensor network, Machine Type Communication (MTC), andMachine-to-Machine (M2M) communication may be implemented bybeamforming, MIMO, and array antennas. Application of a cloud RadioAccess Network (RAN) as the above-described Big Data processingtechnology may also be considered to be as an example of convergencebetween the 5G technology and the IoT technology.

Modern mobile communications increasingly tend to focus on multimediaservices that provide users with high-rate transmission. FIG. 1 is asystem architecture diagram showing System Architecture Evolution (SAE).

A User Equipment (UE) 101 is a terminal equipment for receiving data. AnEvolved Universal Terrestrial Radio Access Network (E-UTRAN) 102 is aradio access network in which a macro eNodeB/NodeB providing the UE withan interface for accessing the radio network is included. A MobilityManagement Entity (MME) 103 is responsible for managing a Mobilitycontext, a session context and security information for the UE. AServing Gateway (SGW) 104 mainly functions to provide a user plane, andthe MME 103 and the SGW 104 may be in a same physical entity. A PacketData Network Gateway (PGW) 105 is responsible for charging, lawfulinterception or more, and the PGW 105 and the SGW 104 may also be in asame physical entity. A Policy and Charging Rules Function Entity (PCRF)106 provides Quality of Service (QoS) policy and charging rules. AServing GPRS Support Node (SGSN) 108 is a network node equipmentproviding routing for data transmission in a Universal MobileTelecommunication System (UMTS). A Home Subscriber Server (HSS) 109 is ahome ownership subsystem of the UE, and is responsible for protectinguser information comprising the current location of the UE, the addressof a serving node, user security information, a packet data context ofthe UE, or more.

FIG. 2 shows an overall architecture of 5G. A User Equipment (UE) 201 isa terminal equipment for receiving data. A Next Generation Radio AccessNetwork (NG-RAN) 102 is a radio access network in which a base stationproviding the UE with an interface for accessing the radio network isincluded. The base station may be a gNB or an eNB connected to 5GC, andthe eNB connected to 5GC may also be called an nb-eNB. An Access Controland Mobility Management Function Entity (AMF) 203 is responsible formanaging a Mobility context and security information for the UE. A UserPlane Function Entity (UPF) 204 mainly provides a user plane function. ASession Management Function Entity (SMF) 205 is responsible for sessionmanagement. A Data Network (DN) 206 contains operator services, theaccess to Internet, third-party services or more.

The interface between the NG-RAN and the AMF is NG-C or N2, and theinterface between the NG-RAN and the UPF is NG-U or N3.

In the next generation network deployment, there is a scenario in whichthe LTE network and the 5G network coexist. When a UE moves in aboundary between the E-UTRAN and the NG-RAN, technologies for realizinga handover between different radio access technologies (inter-RAThandover) are required in order to ensure the continuity of services.Since there are EPS bearers in an EPS but no EPS bearers in a 5GS, thereis a series of problems on how to perform data forwarding during thehandover between the two systems.

SUMMARY

It can be known from the description of the prior art that thearchitecture of 5G is different from that of LTE. In addition, the databearer mode in 5G is also different from that in an LTE system. In theLTE, each Packet Data Network (PDN) connection is divided into multiplebearers according to different Qos levels, and different interfaces andbearers are in one-to-one correspondence. However, in the 5G system, foreach Packet Data Unit (PDU) session on the network side, data istransmitted by one tunnel. At an air interface, data received by a sametunnel from a network side can be mapped to different Data Radio Bearers(DRBs) for transmission. The concept of the PDN connection in the LTEsystem is the same as the concept of the PDU session in the 5G system.These differences need to be completed by a handover process betweendifferent systems. Particularly, at present, it is still not clear howto solve the following problems.

Problem 1: A UE accesses to a second NG-RAN node to establish a PDUsession through a handover in the 5G system, rather than through thetransition from the idle mode to the active mode or through a PDUsession establishment process. Here, the NG-RAN node may be a gNB or anng-eNB. The UE needs to be handed over from the second NG-RAN node to anLTE base station connected to the EPS. How this handover process issupported? Particularly, how data forwarding is performed?

Problem 2: In order to support the continuity of services during thehandover between different systems, and when a UE accesses to the 5GS,when and how an EPS bearer identifier is allocated?

Problem 3: In the case of dual registration, since some PDU sessionsneed to be transferred from the 5GS to the EPS, how data forwarding isperformed?

Problem 4: In the case of dual registration, since some PDU sessionsneed to be transferred from the EPS to the 5GS, how data forwarding isperformed?

Problem 5: A UE accesses to a second eNB to establish a PDU sessionthrough a handover in the EPS system, rather than through the transitionfrom the idle mode to the active mode or through a PDN connectionestablishment process. The UE needs to be handed over from the secondeNB to a base station connected to the 5GS. How this handover process issupported? Particularly, how data forwarding is performed?

The objective of the present invention is to solve at least one of theabove technical deficiencies, particularly the data forwarding problemduring the movement of a UE between an LTE system and a 5G system.

In accordance with an aspect of the present disclosure, an embodiment ofthe present invention provides a method for supporting handover,comprising the following steps of: acquiring, by a first base station ina first communication system, Evolved Packet System (EPS) bearerinformation mapped by a Quality of Service (Qos) flow in a Packet DataUnit (PDU) session through an intra-system handover process;transmitting, by the first base station, a handover required message toan Access and Mobility Function entity (AMF), wherein the handoverrequired message contains the EPS bearer information; receiving, by thefirst base station, a handover command message carrying data forwardingtunnel information transmitted by the AMF; and

performing, by the first base station and with a third base station in asecond communication system, data forwarding according to the receiveddata forwarding tunnel information.

In accordance with another aspect of the present disclosure, anotherembodiment of the present invention provides a method for supportinghandover, comprising the following steps of: receiving, by an Access andMobility Function entity (AMF) in a first communication system, ahandover required message transmitted by a first base station, whereinthe handover required message contains Evolved Packet System (EPS)bearer information mapped by a Quality of Service (Qos) flow in a PacketData Unit (PDU) session; and transmitting, by the AMF, a handovercommand message carrying data forwarding tunnel information to the firstbase station; wherein the EPS bearer information is acquired by thefirst base station through an intra-system handover process.

In accordance with another aspect of the present disclosure, anotherembodiment of the present invention provides a method for supportinghandover, comprising the following steps of: transmitting, by a secondbase station, a handover request message to a first base station,wherein the handover request message contains EPS bearer informationmapped by a Qos flow in a PDU session; and receiving, by the second basestation, a handover request acknowledgment message fed back by the firstbase station.

In accordance with another aspect of the present disclosure, anotherembodiment of the present invention provides a method for supportinghandover, comprising the following steps of: acquiring, by a first basestation in a first communication system, Qos flow informationcorresponding to EPS bearer information through an intra-system handoverprocess; transmitting, by the first base station, a handover requiredmessage to a Mobility Management Entity (MME), wherein the handoverrequired message contains the Qos flow information corresponding to EPSbearer information; receiving, by the first base station, a handovercommand message carrying data forwarding tunnel information transmittedby the MME; and performing, by the first base station and with a thirdbase station in a second communication system, data forwarding accordingto the received data forwarding tunnel information.

In accordance with another aspect of the present disclosure, anotherembodiment of the present invention provides a method for supportinghandover, comprising the following steps of: receiving, by a MobilityManagement Entity (MME) in a second communication system, a handoverrequired message transmitted by a first base station, wherein thehandover required message contains Qos flow information corresponding toEPS bearer information; and transmitting, by the MME, a handover commandmessage carrying data forwarding tunnel information to the first basestation; wherein the Qos flow information is acquired by the first basestation through an intra-system handover process.

In accordance with another aspect of the present disclosure, anotherembodiment of the present invention provides a method for supportinghandover, comprising the following steps of: transmitting, by a secondbase station, a handover request message to a first base station,wherein the handover request message contains Qos flow information;receiving, by the second base station, a handover response message fedback by the first base station; and transmitting, by the base station, ahandover command message carrying data forwarding tunnel information toa UE.

In accordance with another aspect of the present disclosure, anotherembodiment of the present invention provides a base station comprising:an acquisition module configured to acquire, through an intra-systemhandover process, Evolved Packet System (EPS) bearer information mappedby a Quality of Service (Qos) flow in a Packet Data Unit (PDU) session;a transmitting module configured to transmit a handover required messageto an Access and Mobility Function entity (AMF), wherein the handoverrequired message contains the EPS bearer information; a receiving moduleconfigured to receive a handover command message carrying dataforwarding tunnel information transmitted by the AMF; and a forwardingmodule configured to perform data forwarding according to the receiveddata forwarding tunnel information.

In accordance with another aspect of the present disclosure, anotherembodiment of the present invention provides a base station comprising:an acquisition module configured to acquire, through an intra-systemhandover process, Qos flow information corresponding to EPS bearerinformation; a transmitting module configured to transmit a handoverrequired message to a Mobility Management Entity (MME), wherein thehandover required message contains the Qos flow informationcorresponding to EPS bearer information; a receiving module configuredto receive a handover command message carrying data forwarding tunnelinformation transmitted by the MME; and a forwarding module configuredto perform data forwarding according to the received data forwardingtunnel information.

In accordance with another aspect of the present disclosure, a method ofa base station for supporting an inter-system handover from an evolvedpacket system (EPS) system to a 5th generation (5G) system is provided.The method comprises receiving, from an access and mobility managementfunction (AMF), a handover request message including E-UTRAN radioaccess bearer (E-RAB) information and quality of service (Qos) flowinformation, identifying whether a protocol data unit (PDU) session isaccepted or not, determining whether a data forwarding for at least oneQos flow associated with the PDU session is accepted or not based on theE-RAB information and the Qos flow information in case that the PDUsession is accepted, and transmitting, to the AMF, a handover requestacknowledgement message including information on the at least one Qosflow based on the determination.

In accordance with another aspect of the present disclosure, a method ofan access and mobility management function (AMF) for supporting aninter-system handover from an evolved packet system (EPS) system to a5th generation (5G) system is provided. The method comprises receiving,from a session management function (SMF), a first message includingquality of service (Qos) flow information; transmitting, to a basestation, a handover request message including E-UTRAN radio accessbearer (E-RAB) information and the Qos flow information, receiving, fromthe base station, a handover request acknowledgement message includinginformation indicating whether a data forwarding for at least one Qosflow associated with a protocol data unit (PDU) session is accepted ornot, the information being determined based on the E-RAB information andthe Qos flow information, when the PDU session is accepted by the basestation, and transmitting, to the SMF, a second message including tunnelinformation for the data forwarding.

In accordance with another aspect of the present disclosure, a basestation for supporting an inter-system handover from an evolved packetsystem (EPS) system to a 5th generation (5G) system is provided. Thebase station comprises a transceiver and a controller coupled with thetransceiver and configured to receive, from an access and mobilitymanagement function (AMF), a handover request message including E-UTRANradio access bearer (E-RAB) information and quality of service (Qos)flow information, identify whether a protocol data unit (PDU) session isaccepted or not, determine whether a data forwarding for at least oneQos flow associated with the PDU session is accepted or not based on theE-RAB information and the Qos flow information in case that the PDUsession is accepted, and transmit, to the AMF, a handover requestacknowledgement message including information on the at least one Qosflow based on the determination.

In accordance with another aspect of the present disclosure, an accessand mobility management function (AMF) for supporting an inter-systemhandover from an evolved packet system (EPS) system to a 5th generation(5G) system is provided. The AMF comprises a transceiver and acontroller coupled with the transceiver and configured to receive, froma session management function (SMF), a first message including qualityof service (Qos) flow information, transmit, to a base station, ahandover request message including E-UTRAN radio access bearer (E-RAB)information and the Qos flow information, receive, from the basestation, a handover request acknowledgement message includinginformation indicating whether a data forwarding for at least one Qosflow associated with a protocol data unit (PDU) session is accepted ornot, the information being determined based on the E-RAB information andthe Qos flow information, when the PDU session is accepted by the basestation, and transmit, to the SMF, a second message including tunnelinformation for the data forwarding.

By the data forwarding method for supporting handover according to thepresent invention, the data continuity problem during the movementbetween an EPS and a 5GC system is solved, so that the loss of data isavoided and the delay of data forwarding is reduced.

Additional aspects and advantages of the present invention will bepartially appreciated and become apparent from the descriptions below,or will be well learned from the practices of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and/or additional aspects and advantageous of the presentinvention will become apparent and be more readily appreciated from thefollowing descriptions of embodiments, with reference to theaccompanying drawings, in which:

FIG. 1 is a system architecture diagram showing SAE;

FIG. 2 is a schematic diagram of an overall architecture of 5G;

FIG. 3 is a schematic flowchart of the access of a UE to an NG-RAN basestation through an NG handover;

FIG. 4 is a schematic flowchart of a handover of a UE from a 5G systemto an EPS system;

FIG. 5 is a schematic flowchart of the access of a UE to an NG-RAN basestation through an NG handover;

FIG. 6 is a schematic flowchart of the access of a UE to an NG-RAN basestation through an Xn handover;

FIG. 7 is a schematic flowchart of the access of a UE to an NG-RAN basestation through an Xn handover;

FIG. 8 is a schematic flowchart of a first method for acquiring, by anNG-RAN, an EPS bearer identifier when a UE allocates the EPS beareridentifier corresponding to a Qos flow;

FIG. 9 is a schematic flowchart of a second method for acquiring, by anNG-RAN, an EPS bearer identifier when a UE allocates the EPS beareridentifier corresponding to a Qos flow;

FIG. 10 is a schematic flowchart of a method for transferring a PDUsession from a 5G system to an EPS system when a UE is registered inboth the 5G system and the EPS system;

FIG. 11 is a schematic flowchart of a PDU connection establishmentprocess to support a handover between different systems;

FIG. 12 is a schematic flowchart of a UE accessing to an eNB through anS1 handover to support a handover between different systems;

FIG. 13 is a schematic flowchart of a UE to accessing to an eNB throughan X2 handover to support a handover between different systems;

FIG. 14 is a schematic flowchart of a first method for handing over a UEfrom an EPS system to a 5G system;

FIG. 15 is a schematic flowchart of a method for transferring a PDUsession from an EPS to a 5G system when a UE is registered in both the5G system and the EPS;

FIG. 16 is a schematic flowchart of a handover to an EPS when an NG-RANallocates an EPS bearer identifier corresponding to a Qos flow; and

FIG. 17 is a schematic flowchart of a second method for handing over aUE from an EPS system to a 5G system.

DETAILED DESCRIPTION

The objective of the present invention is to solve at least one of theabove technical deficiencies, particularly the data forwarding problemduring the movement of a UE between an LTE system and a 5G system.

An embodiment of the present invention provides a method for supportinghandover, comprising the following steps of:

acquiring, by a first base station in a first communication system,Evolved Packet System (EPS) bearer information mapped by a Quality ofService (Qos) flow in a Packet Data Unit (PDU) session through anintra-system handover process;

transmitting, by the first base station, a handover required message toan Access and Mobility Function entity (AMF), wherein the handoverrequired message contains the EPS bearer information;

receiving, by the first base station, a handover command messagecarrying data forwarding tunnel information transmitted by the AMF; and

performing, by the first base station and with a third base station in asecond communication system, data forwarding according to the receiveddata forwarding tunnel information.

Specifically, the EPS bearer information acquired by the first basestation contains an Evolved Radio Access Bearer (E-RAB) identifier; and

the handover required message contains the identifier of the E-RAB and adownlink data forwarding proposal.

Specifically, the first communication system is a 5G communicationsystem, and the second communication system is an LTE communicationsystem.

Another embodiment of the present invention provides a method forsupporting handover, comprising the following steps of:

receiving, by an Access and Mobility Function entity (AMF) in a firstcommunication system, a handover required message transmitted by a firstbase station, wherein the handover required message contains EvolvedPacket System (EPS) bearer information mapped by a Quality of Service(Qos) flow in a Packet Data Unit (PDU) session; and

transmitting, by the AMF, a handover command message carrying dataforwarding tunnel information to the first base station;

wherein the EPS bearer information is acquired by the first base stationthrough an intra-system handover process.

Specifically, the intra-system handover process comprises the followingsteps of:

receiving, by the AMF, a handover required message transmitted by asecond base station;

transmitting, by the AMF, a PDU handover request message to a SessionManagement Function entity (SMF);

receiving, by the AMF, a PDU handover request response message fed backby the SMF, wherein the PDU handover request response message containsthe EPS bearer information mapped by the Qos flow in the PDU session;

transmitting, by the AMF and to the first base station, a handoverrequest message containing the EPS bearer information mapped by the Qosflow in the PDU session; and

receiving, by the AMF, a handover request acknowledgment message fedback by the first base station, and transmitting a handover commandmessage to a second source base station.

Specifically, the intra-system handover process comprises the followingsteps of:

receiving, by the AMF, a path switch request message transmitted by thefirst base station;

transmitting, by the AMF, an N11 message to the SMF, wherein the N11message contains N2 session information received based on the pathswitch request message;

receiving, by the AMF, an N11 message acknowledgment message transmittedby the SMF, wherein the acknowledgment message contains the EPS bearerinformation mapped by the Qos flow in the PDU session; and

transmitting, by the AMF, a path switch request acknowledgment messageto the first base station, wherein the path switch requestacknowledgment message contains the EPS bearer information mapped by theQos flow in the PDU session received from the AMF.

Correspondingly, still another embodiment of the present inventionprovides a method for supporting handover, comprising the followingsteps of:

transmitting, by a second base station, a handover request message to afirst base station, wherein the handover request message contains EPSbearer information mapped by a Qos flow in a PDU session; and

receiving, by the second base station, a handover request acknowledgmentmessage fed back by the first base station.

Still another embodiment of the present invention provides a method forsupporting handover, comprising the following steps of:

acquiring, by a first base station in a first communication system, Qosflow information corresponding to EPS bearer information through anintra-system handover process;

transmitting, by the first base station, a handover required message toa Mobility Management Entity (MME), wherein the handover requiredmessage contains the Qos flow information corresponding to EPS bearerinformation;

receiving, by the first base station, a handover command messagecarrying data forwarding tunnel information transmitted by the MME; and

performing, by the first base station and with a third base station in asecond communication system, data forwarding according to the receiveddata forwarding tunnel information.

Specifically, the Qos flow information corresponding to EPS bearerinformation acquired by the first base station contains Qos and/or a Qosflow identifier; and

the handover required message contains the PDU session identifier and adownlink data forwarding proposal for the PDU session.

Specifically, the first communication system is a 5G communicationsystem, and the second communication system is an LTE communicationsystem.

Still another embodiment of the present invention provides a method forsupporting handover, comprising the following steps of:

receiving, by a Mobility Management Entity (MME) in a secondcommunication system, a handover required message transmitted by a firstbase station, wherein the handover required message contains Qos flowinformation corresponding to EPS bearer information; and

transmitting, by the MME, a handover command message carrying dataforwarding tunnel information to the first base station;

wherein the Qos flow information is acquired by the first base stationthrough an intra-system handover process.

Specifically, the intra-system handover process comprises the followingsteps of:

receiving, by the MME, a PDU connection request message transmitted by aUE;

transmitting, by the MME, a session establishment request message to aServing Gateway (SGW);

receiving, by the MME, a session establishment response message fed backby the SGW, wherein the session establishment response message containsQos flow information corresponding to an EPS bearer;

transmitting, by the MME, a bearer establishment request message to thefirst base station, wherein the message contains the Qos flowinformation corresponding to the EPS bearer; and

receiving, by the MME, a bearer establishment response message fed backby the first base station.

Specifically, the intra-system handover process comprises the followingsteps of:

receiving, by the MME, handover request information transmitted by asecond base station, wherein the handover request information containsQos flow information; and

transmitting, by the MME, a handover command message carrying dataforwarding tunnel information to the first base station.

Yet another embodiment of the present invention provides a method forsupporting handover, comprising the following steps of:

transmitting, by a second base station, a handover request message to afirst base station, wherein the handover request message contains Qosflow information;

receiving, by the second base station, a handover response message fedback by the first base station; and

transmitting, by the base station, a handover command message carryingdata forwarding tunnel information to a UE.

A base station is provided, comprising:

an acquisition module configured to acquire, through an intra-systemhandover process, Evolved Packet System (EPS) bearer information mappedby a Quality of Service (Qos) flow in a Packet Data Unit (PDU) session;

a transmitting module configured to transmit a handover required messageto an Access and Mobility Function entity (AMF), wherein the handoverrequired message contains the EPS bearer information;

a receiving module configured to receive a handover command messagecarrying data forwarding tunnel information transmitted by the AMF; and

a forwarding module configured to perform data forwarding according tothe received data forwarding tunnel information.

A base station is provided, comprising:

an acquisition module configured to acquire, through an intra-systemhandover process, Qos flow information corresponding to EPS bearerinformation;

a transmitting module configured to transmit a handover required messageto a Mobility Management Entity (MME), wherein the handover requiredmessage contains the Qos flow information corresponding to EPS bearerinformation;

a receiving module configured to receive a handover command messagecarrying data forwarding tunnel information transmitted by the MME; and

a forwarding module configured to perform data forwarding according tothe received data forwarding tunnel information.

In accordance with another aspect of the present disclosure a method ofa base station for supporting an inter-system handover from an evolvedpacket system (EPS) system to a 5th generation (5G) system is provided.The method comprises receiving, from an access and mobility managementfunction (AMF), a handover request message including E-UTRAN radioaccess bearer (E-RAB) information and quality of service (Qos) flowinformation, identifying whether a protocol data unit (PDU) session isaccepted or not, determining whether a data forwarding for at least oneQos flow associated with the PDU session is accepted or not based on theE-RAB information and the Qos flow information in case that the PDUsession is accepted, and transmitting, to the AMF, a handover requestacknowledgement message including information on the at least one Qosflow based on the determination.

The handover request acknowledgement message includes informationindicating that the data forwarding for at least one Qos flow isaccepted in case that the data forwarding for the at least one Qos flowis accepted.

The handover request acknowledgement message includes informationindicating that the data forwarding for at least one Qos flow is notaccepted in case that the data forwarding for the at least one Qos flowis not accepted.

The E-RAB information includes an E-RAB identifier and data forwardinginformation.

The Qos flow information includes information on mapping between anE-RAB and the at least one Qos flow.

The handover request message includes handover type information.

The handover request acknowledgement message includes PDU sessioninformation indicating that the PDU session is rejected to beestablished by the base station and cause information for a rejection incase that the PDU session is not accepted.

In accordance with another aspect of the present disclosure, a method ofan access and mobility management function (AMF) for supporting aninter-system handover from an evolved packet system (EPS) system to a5th generation (5G) system is provided. The method comprises receiving,from a session management function (SMF), a first message includingquality of service (Qos) flow information; transmitting, to a basestation, a handover request message including E-UTRAN radio accessbearer (E-RAB) information and the Qos flow information, receiving, fromthe base station, a handover request acknowledgement message includinginformation indicating whether a data forwarding for at least one Qosflow associated with a protocol data unit (PDU) session is accepted ornot, the information being determined based on the E-RAB information andthe Qos flow information, when the PDU session is accepted by the basestation, and transmitting, to the SMF, a second message including tunnelinformation for the data forwarding.

The Qos flow information includes information on mapping between anE-RAB and the at least one Qos flow.

The handover request acknowledgement message includes the tunnelinformation.

In accordance with another aspect of the present disclosure, a basestation for supporting an inter-system handover from an evolved packetsystem (EPS) system to a 5th generation (5G) system is provided. Thebase station comprises a transceiver and a controller coupled with thetransceiver and configured to receive, from an access and mobilitymanagement function (AMF), a handover request message including E-UTRANradio access bearer (E-RAB) information and quality of service (Qos)flow information, identify whether a protocol data unit (PDU) session isaccepted or not, determine whether a data forwarding for at least oneQos flow associated with the PDU session is accepted or not based on theE-RAB information and the Qos flow information in case that the PDUsession is accepted, and transmit, to the AMF, a handover requestacknowledgement message including information on the at least one Qosflow based on the determination.

The handover request acknowledgement message includes informationindicating that the data forwarding for at least one Qos flow isaccepted in case that the data forwarding for the at least one Qos flowis accepted.

The handover request acknowledgement message includes informationindicating that the data forwarding for at least one Qos flow is notaccepted in case that the data forwarding for the at least one Qos flowis not accepted.

The handover request acknowledgement message includes PDU sessioninformation indicating that the PDU session is rejected to beestablished by the base station and cause information for a rejection incase that the PDU session is not accepted.

In accordance with another aspect of the present disclosure, an accessand mobility management function (AMF) for supporting an inter-systemhandover from an evolved packet system (EPS) system to a 5th generation(5G) system is provided. The AMF comprises a transceiver and acontroller coupled with the transceiver and configured to receive, froma session management function (SMF), a first message including qualityof service (Qos) flow information, transmit, to a base station, ahandover request message including E-UTRAN radio access bearer (E-RAB)information and the Qos flow information, receive, from the basestation, a handover request acknowledgement message includinginformation indicating whether a data forwarding for at least one Qosflow associated with a protocol data unit (PDU) session is accepted ornot, the information being determined based on the E-RAB information andthe Qos flow information, when the PDU session is accepted by the basestation, and transmit, to the SMF, a second message including tunnelinformation for the data forwarding.

Embodiments of the present invention will be described in detailhereinafter. The examples of these embodiments have been illustrated inthe accompanying drawings throughout which same or similar referencenumerals refer to same or similar elements or elements having same orsimilar functions. The embodiments described with reference to theaccompanying drawings are illustrative, merely used for explaining thepresent invention and should not be regarded as any limitations thereto.

The present invention provides a method for supporting handover,comprising the following steps of:

acquiring, by a first base station in a first communication systemEvolved Packet System (EPS) bearer information mapped by a Quality ofService (Qos) flow in a Packet Data Unit (PDU) session through anintra-system handover process;

transmitting, by the first base station, a handover required message toan Access and Mobility Function entity (AMF), wherein the handoverrequired message contains the EPS bearer information;

receiving, by the first base station, a handover command messagecarrying data forwarding tunnel information transmitted by the AMF; and

performing, by the first base station and with a third base station in asecond communication system, data forwarding according to the receiveddata forwarding tunnel information.

Specifically, the first communication system is a 5G communicationsystem, and the second communication system is an LTE communicationsystem.

Correspondingly, the present invention further provides a method forsupporting handover, comprising the following steps of:

receiving, by an Access and Mobility Function entity (AMF) in a firstcommunication system, a handover required message transmitted by a firstbase station, wherein the handover required message contains EvolvedPacket System (EPS) bearer information mapped by a Quality of Service(Qos) flow in a Packet Data Unit (PDU) session; and

transmitting, by the AMF, a handover command message carrying dataforwarding tunnel information to the first base station;

wherein the EPS bearer information is acquired by the first base stationthrough an intra-system handover process.

The method for supporting handover will be described below in detail byspecific embodiments.

As shown in FIG. 3 , the method is used by an NG-RAN node to acquire theinformation for inter-system handover in order to support the scenariowhen a UE accesses to an NG-RAN node through an NG handover procedureand the NG-RAN node needs to trigger an inter-system handover to EPS.Here, the NG-RAN node can be a gNB or an eNB connected to the 5GC (alsocalled an ng-eNB). This method comprises the following steps.

Step 301: A source NG-RAN node (S-NG-RAN node) decides to initiate ahandover of a UE.

Step 302: The S-NG-RAN node transmits a handover required message to anAMF.

The UE has one or more ongoing PDU sessions. Each PDU session containsone or more Qos flows. During a PDU session establishment or GuaranteedBusiness Rate (GBR) Qos flow establishment process, mapped EPS Qosinformation and/or an EPS bearer identifier is allocated to a Qos flow.A Non-GBR Qos flow is mapped to a default EPS bearer. A GBR Qos flow ismapped to a dedicated EPS bearer. EPS Qos information mapped by a Qosflow can be allocated by a PCC or an SMF. The EPS bearer identifiercorresponding to a QoS flow can be allocated by an SMF, an AMF or a UE.In order to support the handover between different systems, an SMF canhave functions of a PGW control plane. In a PCC deployment scenario, aPolicy Control Function (PCF) provides the SMF with mapped EPS Qos of aQos flow. In order to support the handover between different systems,the PCF can have a Policy Control and Charging Rules Function (PCRF).The SMF transmits, through the AMF and to the UE, the mapped EPS Qosinformation and/or an EPS bearer identifier of a Qos flow. For example,the SMF transmits the information to the UE through a non-access stratummessage a PDU session establishment message. During the PDU sessionestablishment or GBR Qos flow establishment process, the SMF can furthertransmit, through the AMF and to an NG-RAN, the mapped EPS Qosinformation and/or an E-RAB identifier of a Qos flow. The AMF transmitsthe mapped EPS Qos information and/or an E-RAB identifier of a Qos flowto the NG-RAN through an initial context establishment request messageor a PDU session resource establishment request message. The NG-RAN cantransmit the mapped EPS Qos information and/or an E-RAB identifier of aQos flow to the UE through an RRC message. The E-RAB identifier and theEPS bearer identifier are identical or in one-to-one correspondence. Itis called an EPS bearer in a core network, while it is called an E-RABin an access network. The E-RAB identifier and the EPS bearer identifierare identical or in one-to-one correspondence. It is called the EPSbearer identifier in a core network, while it is called the E-RABidentifier in an access network.

In a source-to-target transparent transmitter, the S-NG-RAN nodecontains the EPS bearer identifier and/or EPS Qos information mapped bya Qos flow in the PDU session.

Step 303: The AMF transmits a PDU handover request message to the SMF.This message is specific to each PDU session on which an NG handover isto be performed. This message contains a PDU session identifier and atarget identifier.

Step 304: The SMF transmits a PDU handover response message to the AMF.This message contains a PDU session identifier and information ofSession Management (SM) over an NG-C interface. The SMF selects an UPFhaving an interface to a target gNB.

Step 305: The AMF detects the PDU handover response message of each SMF.When the AMF receives all PDU handover responses or the maximum waitingtime arrives, the handover process will be continuously executed.

In the method of the present invention, steps 303 to 305 are notmandatory steps. The PDU session information and the information of theQos flows contained in the PDU session can be contained in the handoverrequired message, so that it is not necessary to execute steps 303 to305.

Step 306: The AMF transmits a handover request message to a targetNG-RAN node (T-NG-RAN node). This message contains a source-to-targettransparent transmitter. The source-to-target transparent transmittercontains the EPS bearer identifier and/or EPS Qos information mapped bya Qos flow in the PDU session. The handover request further contains PDUsession information to be established. The PDU session information to beestablished contains a PDU session identifier.

When the target NG-RAN node needs to initiate a handover of the UE to anEPS system, the target NG-RAN node uses the received EPS beareridentifier and/or EPS Qos information mapped by a Qos flow in the PDUsession so that the data forwarding during the handover betweendifferent systems can be supported. The specific process is as describedin the embodiment shown in FIG. 4 .

Step 307: The T-NG-RAN node transmits a handover request acknowledgementmessage to the AMF. This message contains the data forwarding tunnelinformation allocated by the target base station. This message containsa target-to-source transparent transmitter. This message contains a listof unsuccessfully established PDU sessions.

Step 308: The AMF transmits a PDU modification request message to theSMF. This message contains the indirect data forwarding tunnelinformation allocated by the T-NG-RAN node. The AMF can transmit,through the PDU modification request message, the indirect dataforwarding tunnel information allocated by the T-NG-RAN node and thenrequest to allocate tunnel information used for data forwarding from theS-NG-RAN node to the UPF, or the AMF can transmit, through anotherindividual message, the indirect data forwarding tunnel informationallocated by the T-NG-RAN node and then request to allocate tunnelinformation used for data forwarding from the S-NG-RAN node to the UPF.

Step 309: The SMF transmits a PDU modification response message to theAMF.

Step 310: The AMF transmits a handover command message to the S-NG-RANnode. This message contains a target-to-source transparent transmitter.For the indirect data forwarding, the AMF transmits, to the source basestation, the data forwarding tunnel information allocated by the UPF. Ifthe UPF changes during a handover process, the AMF transmits, to thesource base station, the data forwarding tunnel information allocated bythe source UPF.

Step 311: The S-NG-RAN node transmits a handover command message to theUE. The UE is synchronized to the target base station.

Step 312: The UE transmits a handover completion message to the T-NG-RANnode.

Step 313: The T-NG-RAN node transmits a handover notification message tothe AMF.

Step 314: The AMF transmits a handover completion message to the SMF.This message contains the PDU session identifier. This message isspecific to each PDU session.

Step 315: The SMF transmits a handover completion acknowledgementmessage to the AMF. The SMF needs to inform the selected UPF ofinformation indicating that the downlink user plane tunnel is handedover to the T-NG-RAN node, i.e., the downlink tunnel informationallocated by the T-NG-RAN node.

Step 316: The AMF transmits a UE context release command message to theS-NG-RAN node.

Step 317: The S-NG-RAN node transmits a UE context release completionmessage to the AMF.

So far, the first method for supporting handover according to thepresent invention has been described. By this method, the dataforwarding can be well supported when a UE accesses to an NG-RAN nodethrough a handover and when the NG-RAN node needs to initiate a handoverof the UE from a 5GS to an EPS, so that the loss of data is avoided, thedelay of the data forwarding is reduced and the continuity of servicesis ensured.

As shown in FIG. 4 , this method can well support the data forwardingduring the handover between different systems. This method comprises thefollowing steps.

Step 401: An NG-RAN decides to hand over a UE to an E-UTRAN.

Here, the E-UTRAN can be an eNB connected to the EPC. The NG-RAN can bea gNB, an eNB connected to the 5GC or a Centralized Unit (CU) in thegNB.

The user plane path before handover is from an UPF to the NG-RAN. TheSGW needs to support an interface to the UPF. The UPF can contain afunction of a PGW user plane and a function of performing user planeanchoring during the handover between different RATs.

The UE has one or more ongoing PDU sessions. Each PDU session containsone or more Qos flows. During a PDU session establishment or GuaranteedBusiness Rate (GBR) Qos flow establishment process, mapped EPS Qosinformation and/or EPS bearer identifier is allocated to a Qos flow. ANon-GBR Qos flow is mapped to a default EPS bearer. A GBR Qos flow ismapped to a dedicated EPS bearer. EPS Qos information mapped by a Qosflow can be allocated by a PCC or an SMF. The EPS bearer identifiermapped by a Qos flow can be allocated by an SMF, an AMF or a UE. Inorder to support the handover between different systems, the SMF canhave functions of a PGW control plane. In a PCC deployment scenario, aPolicy Control Function (PCF) provides the SMF with mapped EPS Qos of aQos flow. In order to support the handover between different systems,the PCF can have a Policy Control and Charging Rules Function (PCRF).The SMF transmits, through the AMF and to the UE, the mapped EPS Qosinformation and/or an EPS bearer identifier of a Qos flow. For example,the SMF transmits the information to the UE through a non-access stratummessage PDU session establishment message. During the PDU sessionestablishment or GBR Qos flow establishment process, the SMF can furthertransmit, through the AMF and to an NG-RAN, the mapped EPS Qosinformation and/or an E-RAB identifier of a Qos flow. The AMF transmitsthe mapped EPS Qos information and/or an E-RAB identifier of a Qos flowto the NG-RAN through an initial context establishment request messageor a PDU session resource establishment request message. The NG-RAN cantransmit the mapped EPS Qos information and/or an E-RAB identifier of aQos flow to the UE through an RRC message. The E-RAB identifier and theEPS bearer identifier are identical or in one-to-one correspondence. Itis called an EPS bearer in a core network, while it is called an E-RABin an access network. The E-RAB identifier and the EPS bearer identifierare identical or in one-to-one correspondence. It is called the EPSbearer identifier in a core network, while it is called the E-RABidentifier in an access network.

Step 402: The NG-RAN transmits a handover required message to an AMF.This message contains the identifier of a target eNB and asource-to-target transparent transmitter. The source-to-targettransparent transmitter contains the E-RAB identifier and a downlinkdata forwarding proposed for an E-RAB. The NG-RAN acquires, through aPDU session establishment process or a handover process, the EPS beareridentifier mapped by a Qos flow in the PDU session. It is also possibleto acquire the mapped EPS Qos information. The method for acquiring theEPS bearer information by the NG-RAN through handover is as described inthe method shown in FIGS. 3, 5, 6 and 7 . The method for acquiring theEPS bearer information through a PDU session establishment process is asdescribed in step 401 or the method shown in FIGS. 8 and 9 . The NG-RANdecides, according to the mapping from the Qos flow to the E-RAB and theQos information, whether to propose data forwarding. The NG-RAN can makea decision by considering other factors, for example, the presence orabsence of data in a buffer, without influencing the main contents ofthe present invention.

The handover required message can further contain a list of EPS bearerinformation. The EPS bearer information contains an EPS beareridentifier and Qos information of the EPS bearer.

This message further contains identification information indicating anMME to which the target eNB is connected. The identification informationcan be an identifier of a tracked area or an identifier of the MME.

The NG-RAN informs the AMF of the handover type. The handover typeincludes a handover in NR, a handover from NR to LTE, a handover from NRto UTRAN and a handover from NR to GERAN and/or GSM. For the handoverfrom NR to LTE, the NG-RAN informs the AMF whether the target basestation for the handover is a base station connected to a 5G corenetwork or whether this handover is a handover between differentsystems. An LTE eNB may support an interface to the 5GC, or may notsupport an interface to the 5GC. If the target base station is alsoconnected to the 5GC, the handover is a handover in the 5G system. Ifthe target base station is connected to the EPC but not connected to the5GC, the handover is a handover between different systems. The NG-RANcan contain, in the handover required message, information about aninter-system handover or information indicating that there is nointerface between the target base station and the 5GC, so as to informthe AMF that this handover is a handover between different systems. Ifan identifier of an eNB connected to the 5GC and an identifier of an eNBnot connected to the 5GC are different in length, the 5GC can determine,according to the length of the identifier of the target base stationcontained in the received handover required message, whether thehandover is a handover between different systems. If the identificationinformation of the MME connected to the target base station and theidentifier of the AMF node are defined differently (for example,different in length), the 5GC can determine, according to the length ofthe identifier of the core network connected to the target base stationcontained in the received handover required message, whether thehandover is a handover between different systems. Or, the NG-RANdirectly sets the handover type in the handover required message as ahandover from the NR to an eNB connected to the EPC or a handover fromthe NR to an eNB connected to the 5GC, to inform the AMF of the handovertype. The handover type indicates the core network to which the targeteNB is connected is an EPC or a 5GC. If the core network is a 5GC, thehandover is an intra-system handover. If the core network is an EPC, thehandover is a handover between different systems.

Step 403 a: The AMF transmits a Session Management (SM) context requestmessage to an SMF. According to the received information in the handoverrequired message, the AMF knows that the handover is a handover betweendifferent systems, and the AMF requests the SMF to provide an SMcontext. The AMF can further request for an EPS bearer context. The AMFtransmits this message to each SMF serving the UE.

Step 403 b: The SMF transmits an SM context response message to the AMF.This message contains the SM context of the UE. The SM context furthercontains the mapped EPS bearer context, for example, the EPS beareridentifier and/or EPS Qos information. When the AMF requests for the SMcontext, the SMF also always feeds back the mapped EPS bearer context tothe AMF if there is mapped EPS bearer context. Or, if the AMF alsorequests for the mapped EPS bearer context when it requests for the SMcontext to the SMF, the SMF transmits the mapped EPS bearer context tothe SMF. According to the information, which is received from the sourceNG-RAN, indicating that there is no connection between the target eNBand the 5G core network or indicating that the handover is a handoverbetween different systems or a handover from the NR to an eNB connectedto the EPC, the AMF knows that the handover is a handover betweendifferent systems, and the AMF requests the SMF to provide the mappedEPS bearer context information.

In this method of the present invention, steps 403 a and 403 b may notbe executed. The AMF acquires, from the handover required messagereceived from the NG-RAN, EPS bearer information in the PDU session, forexample, the EPS bearer identifier and EPS Qos information, so that theAMF can constitute a relocation request message in step 404.

In this method of the present invention, steps 403 a and 403 b may notbe executed. The AMF acquires, from the handover required messagereceived from the NG-RAN, EPS bearer information in the PDU session, forexample, the EPS bearer identifier and EPS Qos information, so that theAMF can constitute a relocation request message in step 404.

Step 404: The AMF transmits a relocation request message to an MME.According to the identification information indicating the MME connectedto the target eNB contained in the handover required message, the AMFselects and finds an MME. The identification information of the MMEconnected to the target eNB can be a TAI. This message contains theidentifier of the target eNB, a source-to-target transparent transmitterand mapped EPS UE context information. The mapped EPS UE contextinformation contains UE Mobility Management (MM) context information andSession Management (SM) context information.

The AMF or the MME decides whether the data forwarding is possible.Here, the data forwarding refers to indirect data forwarding. If the AMFdecides that the indirect data forwarding is impossible, the AMF informsthe MME of the information.

The AMF informs the MME of the information of the Qos flow(s) containedin the PDU session.

Step 405: The MME transmits a session establishment request message toan SGW. This message contains the EPS bearer context information.

Step 406: The SGW transmits a session establishment response message tothe MME. This message contains tunnel information for uplink datatransmission over an S1 interface allocated by the SGW.

Step 407: The MME transmits a handover request message to the E-UTRAN.This message contains a source-to-target transparent transmitter and anE-EAB context. The E-RAB context contains an E-RAB to be established anduplink tunnel information over the S1 interface allocated by the SGW.The E-RAB context contains information indicating whether the dataforwarding is possible. This message contains the handover type. Thespecific content is the same as that in step 402 and will not berepeated here.

Step 408: The E-UTRAN transmits a handover request acknowledgementmessage to the MME. This message contains a list of established E-RABs,a list of unsuccessfully established E-RABs and a target-to-sourcetransparent transmitter. For the established RABs, this message furthercontains tunnel information for downlink data transmission over the S1interface. For the established E-RABs, if the source base station givesa proposal of downlink data forwarding, the data forwarding is possibleand the target eNB accepts the downlink data forwarding proposal, thetarget base station contains the tunnel information which is allocated,by the E-UTRAN to each E-RAB requiring downlink data forwarding, fordata forwarding over the S1 interface.

Step 409: The MME requests the SGW to establish an indirect dataforwarding tunnel. This step is executed only when the indirect dataforwarding is to be executed. Upon receiving, from the E-UTRAN, thedownlink tunnel information for data forwarding over the S1 interface,the MME requests the SGW to establish an indirect data forwardingtunnel. The MME transmits, to the SGW, the address of a transport layerfor data forwarding and a TEID allocated by the eNB. The address of thetransport layer and the TEID correspond to each E-RAB.

The SGW transmits an indirect data forwarding tunnel establishmentresponse message to the MME. This message contains the information fordata forwarding between the SGW and the UPF allocated by the SGW. Theinformation for data forwarding between the SGW and the UPF contains aPDU session identifier and/or E-RAB information contained in the PDUsession. The E-RAB information contains an E-RAB identifier and thetunnel information used by the E-RAB for data forwarding. The tunnelinformation contains the address of a transport layer and a TEIDallocated by the SGW. The SGW allocates downlink data forwarding tunnelinformation to an E-RAB requiring downlink data forwarding. The SGWallocates uplink data forwarding tunnel information to an E-RABrequiring uplink data forwarding. The data forwarding tunnel informationcontained in the E-RAB information can contain uplink and/or downlinkdata forwarding tunnel information.

There are two methods for performing data forwarding between the UPF andthe SGW.

A way for data transmission between the UPF and the SGW is that each EPSin each PDU session bears one tunnel. If the NG-RAN and the UPF performdata forwarding in such a way that each PDU session corresponds to oneuser plane tunnel, the UPF transmits, to the SGW and through a tunnelcorresponding to an EPS bearer of each Qos flow, data of multiple Qosflows belonging to a same PDU session received from the NG-RAN, that is,the UPF performs a mapping from one tunnel to multiple tunnels. The UPFperforms, according to the correspondence between a Qos flow in the PDUsession and an EPS bearer or an E-RAB, a mapping from one tunnel tomultiple tunnels. In this forwarding method, the SGW allocates, to eachEPS bearer requiring data forwarding in each PDU session, a tunnelinformation used for data forwarding between the SGW and the UPF. Foreach PDU session, the number of data forwarding tunnels is equal to thenumber of EPS bearers. The UPF knows, according to the informationreceived from the SMF in step 412, the number of EPS bearers requiringdata forwarding in each PDU session in the EPS, the mapping between aQos flow and an EPS bearer and/or the EPS bearer identifier. The UPFknows information of the Qos flow(s) of the PDU session contained in the5G system.

The step 409 is executed when the indirect data forwarding is possible.

Step 410: The MME transmits a relocation response message to the AMF.This message contains the tunnel information allocated by the SGW fordata forwarding. The tunnel information is specific to each EPS bearerin each PDU session. This message contains a target-to-sourcetransparent transmitter. The MME transmits, to the AMF, the E-RABinformation contained in the PDU session and the tunnel information fordata forwarding allocated by the SGW to each E-RAB.

This message contains the information for data forwarding between theSGW and the UPF allocated by the SGW. The information for dataforwarding between the SGW and the UPF contains a PDU session identifierand/or E-RAB information contained in the PDU session. The E-RABinformation contains an E-RAB identifier and the tunnel information usedby the E-RAB for data forwarding. The information for data forwardingbetween the SGW and the UPF contains the tunnel information for dataforwarding allocated to each E-RAB of each PDU session by the SGW. Thetunnel information for data forwarding can contain uplink and/ordownlink data forwarding tunnel information.

The MME directly transmits the E-RAB information to the AMF, and theE-RAB information is transferred by the AMF.

Step 411: The AMF requests the SMF to create a data forwarding tunnel.The AMF transmits a indirect data forwarding tunnel establishmentrequest message to the SMF. This message contains PDU sessioninformation. The PDU session information contains the PDU sessionidentifier, information of the Qos flow(s) contained in the PDU session,the number of EPS bearers requiring data forwarding in each PDU sessionin the EPS, the mapping between a Qos flow and an EPS bearer, the EPSbearer identifier and/or Qos information of each EPS bearer. Thismessage contains the information for data forwarding received from theMME.

Step 412: The SMF transmits an N4 session modification message to theUPF. This message contains PDU session information. The PDU sessioninformation contains the PDU session identifier, information of the Qosflow(s) contained in the PDU session, the number of EPS bearersrequiring data forwarding in each PDU session in the EPS, the mappingbetween a Qos flow and an EPS bearer, the EPS bearer identifier and/orQos information of each EPS bearer. This message contains theinformation for data forwarding received from the AMF.

The N4 session modification message contains EPS bearer informationcontained in the PDU session. The EPS bearer information contains an EPSbearer identifier and tunnel information used by each EPS bearer fordata forwarding. The SMF informs the UPF of the correspondence between aQos flow and an EPS bearer in the PDU session. The UPF knows the Qosflow information of the PDU session in the 5G system, and the UPFreceives, from the SMF, the EPS bearer information contained in the PDUsession and the mapping between a Qos flow and an EPS bearer.

The UPF allocates tunnel information for data forwarding between theNG-RAN and the UPF and then transmits the tunnel information to the SMF.

A way for data forwarding between the NG-RAN and the UPF is as follows.

The NG-RAN and the UPF perform data forwarding in such a way that eachPDU session corresponds to one user plane tunnel. In this dataforwarding method, the UPF allocates tunnel information for each PDUsession. The tunnel information contains the address of a transportlayer and a TEID.

The UPF transmits the allocated tunnel information for data forwardingto the SMF. The SMF receives an N4 session modification response messagefrom the UPF. This message contains the tunnel information for dataforwarding between the NG-RAN and the UPF allocated by the UPF.

Step 413: The SMF transmits a indirect data forwarding tunnelestablishment response message to the AMF. This message contains thetunnel information for data forwarding between the NG-RAN and the UPFallocated by the UPF.

Step 414: The AMF transmits a handover command message to the NG-RAN.This message contains a target-to-source transparent transmitter and thetunnel information for data forwarding allocated by the UPF. Thismessage further contains information about established PDU sessions andinformation about unsuccessfully established PDU sessions. Theinformation about established PDU sessions contains information aboutestablished Qos flows and information about unsuccessfully establishedQos flows. The tunnel information for data forwarding is specific toeach PDU session.

Step 415: The NG-RAN transmits a handover command message to the UE.

The NG-RAN forwards data to the UPF. For a PDU session requiring dataforwarding and on a corresponding tunnel, the NG-RAN forwards data tothe UPF.

The NG-RAN transmits, on a user plane tunnel allocated for the PDUsession, data of each Qos flow to the UPF. For the downlink data, theNG-RAN transmits, on a tunnel allocated for downlink data forwarding, adownlink data packet to the UPF.

The UPF forwards data to the SGW. The UPF directly forwards the datareceived from the NG-RAN to the SGW through a user plane tunnelallocated for the corresponding EPS bearer. The SGW directly forwardsthe data to the target base station. The UPF forwards, according to themapping between a Qos flow and an EPS bearer and through the user planetunnel allocated for the corresponding EPS bearer, data of different Qosflows in the PDU session to the SGW. According to the mapping between aQos flow and an EPS bearer and the information about the EPS beareraccepting the data forwarding, the UPF knows a Qos flow accepting thedata forwarding, and the UPF forwards, to the SGW and through the userplane tunnel allocated for the corresponding EPS bearer, the data of theQos flow that accepts the data forwarding. Correspondingly, if there isno Qos flow accepting the data forwarding and there is no correspondingdata forwarding tunnel, the UPF discards the data. The SGW directlyforwards data to the target base station.

The SGW forwards data to the E-UTRAN. The SGW transmits, to the E-UTRANand through the corresponding tunnel allocated by the E-UTRAN, the datareceived from the UPF by the tunnel corresponding to each EPS bearer,that is, the UPF performs a mapping from multiple tunnels to one tunnel.The SGW forwards data to the E-UTRAN according to the sessiontransmission method in the EPS.

During the PDU session establishment or GBR Qos flow establishmentprocess, the UE receives, from the network, mapped EPS Qos informationand/or EPS bearer identification information mapped by a Qos flow. TheUE correlates an ongoing Qos flow with the EPS bearer identifiercontained in the handover command message. For a Qos flow without thecorresponding EPS bearer, the UE can delete this Qos flow.

Step 416: The UE transmits a handover completion message to the E-UTRAN.

Step 417: The E-UTRAN transmits a handover completion message to theMME. This message contains the tunnel information allocated by theE-UTRAN for downlink data transmission.

Step 418: The MME transmits a bearer modification request message to theSGW. This message contains the tunnel information for downlink datatransmission over the S1 interface.

Step 419: The SGW transmits a bearer modification request message to theSMF. The SMF can further have a function of a PGW control plane. The SGWallocates tunnel information for downlink data transmission between theSGW and the UPF, and the tunnel information corresponds to each EPSbearer or each PDU session.

Step 420: The SMF requests the UPF to perform session modification. TheSMF can further have a function of a PGW control plane. The SMFtransmits, to the UPF, the tunnel information for downlink datatransmission between the SGW and the UPF allocated by the SGW, and thetunnel information corresponds to each EPS bearer or each PDU session.The UPF transmits a session modification response to the SMF. The UPFallocates tunnel information for uplink data transmission between theSGW and the UPF, and the UPF transmits, to the SMF, the tunnelinformation for uplink data transmission.

Step 421: The SMF transmits a bearer modification response message tothe SGW. This message contains the tunnel information for uplink datatransmission between the SGW and the UPF allocated by the UPF.

Step 422: The SGW transmits a bearer modification response message tothe MME.

So far, the process of supporting the data forwarding during thehandover between different systems can be well supported by using theinformation obtained from intra-5GS handover procedure in the presentinvention. By this method, the handover problem from the 5GS to the EPScan be solved, so that the loss of data is avoided and the continuity ofservices is ensured.

As shown in FIG. 5 , the method is used to support a handover when a UEaccesses to an NG-RAN node through an NG handover process and the NG-RANnode needs to initiate a handover of the UE to an EPS. Here, the NG-RANnode can be a gNB or an eNB connected to the 5GC (also called anng-eNB). This method comprises the following steps.

Step 501: A source NG-RAN node (S-NG-RAN node) decides to initiate ahandover of a UE.

Step 502: The S-NG-RAN node transmits a handover required message to anAMF.

The UE has one or more ongoing PDU sessions. Each PDU session containsone or more Qos flows. During a PDU session establishment or GuaranteedBusiness Rate (GBR) Qos flow establishment process, mapped EPS Qosinformation and/or an EPS bearer identifier is allocated to a Qos flow.A Non-GBR Qos flow is mapped to a default EPS bearer. A GBR Qos flow ismapped to a dedicated EPS bearer. EPS Qos information mapped by a Qosflow can be allocated by a PCC or an SMF. The EPS bearer identifiercorresponding to a QoS flow can be allocated by an SMF, an AMF or a UE.In order to support the handover between different systems, an SMF canhave functions of a PGW control plane. In a PCC deployment scenario, aPolicy Control Function (PCF) provides the SMF with mapped EPS Qos of aQos flow. In order to support the handover between different systems,the PCF can have a Policy Control and Charging Rules Function (PCRF).The SMF transmits, through the AMF and to the UE, the mapped EPS Qosinformation and/or an EPS bearer identifier of a Qos flow. For example,the SMF transmits the information to the UE through a non-access stratummessage PDU session establishment message. During the PDU sessionestablishment or GBR Qos flow establishment process, the SMF can furthertransmit, through the AMF and to an NG-RAN, the mapped EPS Qosinformation and/or an E-RAB identifier of a Qos flow. The AMF transmitsthe mapped EPS Qos information and/or an E-RAB identifier of an Qos flowto the NG-RAN through an initial context establishment request messageor a PDU session resource establishment request message. The NG-RAN cantransmit the mapped EPS Qos information and/or an E-RAB identifier of aQos flow to the UE through an RRC message. The E-RAB identifier and theEPS bearer identifier are identical or in one-to-one correspondence. Itis called an EPS bearer in a core network, while it is called an E-RABin an access network. The E-RAB identifier and the EPS bearer identifierare identical or in one-to-one correspondence. It is called the EPSbearer identifier in a core network, while it is called the E-RABidentifier in an access network.

Step 503: The AMF transmits a PDU handover request message to an SMF.This message is specific to each PDU session on which an NG handover isrequired to be performed. This message contains a PDU session identifierand a target identifier.

Step 504: The SMF transmits a PDU handover response message to the AMF.This message contains a PDU session identifier and information ofSession Management (SM) over an NG-C interface. The SMF selects an UPFhaving an interface to a target gNB. This message contains a mapped EPSbearer identifier and/or EPS Qos information of a Qos flow in the PDUsession.

Step 505: The AMF detects the PDU handover response message from eachSMF. When the AMF receives all PDU handover responses or the maximumwaiting time arrives, the handover process will be continuouslyexecuted.

Step 506: The AMF transmits a handover request message to a targetNG-RAN node (T-NG-RAN node). This message contains a source-to-targettransparent transmitter. This message contains a mapped EPS beareridentifier and/or EPS Qos information of a Qos flow in the PDU session.The handover request further contains PDU session information to beestablished. The PDU session information to be established contains aPDU session identifier. In this method of the present invention, the EPSbearer identifier can be allocated by the SMF, the AMF or the UE. If theEPS bearer identifier is allocated by the SMF, the EPS bearer identifieris contained in both steps 504 and 506. If the EPS bearer identifier isallocated by the UE, during the PDU session establishment process, theSMF acquires, through the AMF and from the UE, the EPS bearer identifiermapped by a Qos flow, and the SMF transmits the EPS bearer identifier tothe target NG-RAN node in steps 504 and 506. If the EPS beareridentifier is allocated by the AMF, the AMF transmits the EPS beareridentifier to the target NG-RAN node in step 506. Or, during the PDUsession establishment process, the AMF transmits the allocated EPSbearer identifier mapped by a Qos flow to the SMF; and during thehandover process, the SMF transmits the EPS bearer identifier to thetarget NG-RAN node in steps 504 and 506.

When the target NG-RAN node needs to initiate a handover of the UE to anEPS system, the target NG-RAN node uses the received EPS beareridentifier and/or EPS Qos information mapped by a Qos flow in the PDUsession so that the data forwarding during the handover betweendifferent systems can be supported. The specific process is as describedin the embodiment shown in FIG. 4 .

Step 507: The T-NG-RAN node transmits a handover request acknowledgementmessage to the AMF. This message contains the data forwarding tunnelinformation allocated by the target base station. This message containsa target-to-source transparent transmitter. This message contains a listof unsuccessfully established PDU sessions.

Step 508: The AMF transmits a PDU modification request message to theSMF. This message contains the indirect data forwarding tunnelinformation allocated by the T-NG-RAN node. The AMF can transmit,through the PDU modification request message, the indirect dataforwarding tunnel information allocated by the T-NG-RAN node and thenrequest to allocate tunnel information used for data forwarding from theS-NG-RAN node to the UPF, or the AMF can transmit, through anotherindividual message, the indirect data forwarding tunnel informationallocated by the T-NG-RAN node and then request to allocate tunnelinformation used for data forwarding from the S-NG-RAN node to the UPF.

Step 509: The SW′ transmits a PDU modification response message to theAMF.

Step 510: The AMF transmits a handover command message to the S-NG-RANnode. This message contains a target-to-source transparent transmitter.For the indirect data forwarding, the AMF transmits, to the source basestation, the data forwarding tunnel information allocated by the UPF. Ifthe UPF changes during a handover process, the AMF transmits, to thesource base station, the data forwarding tunnel information allocated bythe source UPF.

Step 511: The S-NG-RAN node transmits a handover command message to theUE. The UE is synchronized to the target base station.

Step 512: The UE transmits a handover completion message to the T-NG-RANnode.

Step 513: The T-NG-RAN node transmits a handover notification message tothe AMF.

Step 514: The AMF transmits a handover completion message to the SMF.This message contains the PDU session identifier. This message isspecific to each PDU session.

Step 515: The SMF transmits a handover completion acknowledgementmessage to the AMF. The SMF needs to inform the selected UPF ofinformation indicating that the downlink user plane tunnel is handedover to the T-NG-RAN node, i.e., the downlink tunnel informationallocated by the T-NG-RAN node.

Step 516: The AMF transmits a UE context release command message to theS-NG-RAN node.

Step 517: The S-NG-RAN node transmits a UE context release completionmessage to the AMF.

So far, the third method for supporting handover according to thepresent invention has been described. By this method, the dataforwarding can be well supported when a UE accesses to an NG-RAN nodethrough a handover and when the NG-RAN node needs to initiate a handoverof the UE from a 5GS to an EPS, so that the loss of data is avoided, thedelay of the data forwarding is reduced and the continuity of servicesis ensured.

As shown in FIG. 6 , the method is used by an NG-RAN node to acquire theinformation for inter-system handover in order to support the scenariowhen a UE accesses to an NG-RAN node through an Xn handover procedureand the NG-RAN node needs to initiate a handover of the UE to an EPS.Here, the NG-RAN node can be a gNB or an eNB connected to the 5GC (alsocalled an ng-eNB). This method comprises the following steps.

Step 601: A source NG-RAN node (S-NG-RAN node) decides to initiate ahandover of a UE.

Step 602: The S-NG-RAN node transmits a handover request message to atarget NG-RAN node (T-NG-RAN node).

The UE has one or more ongoing PDU sessions. Each PDU session containsone or more Qos flows. During a PDU session establishment or GuaranteedBusiness Rate (GBR) Qos flow establishment process, mapped EPS Qosinformation and/or an EPS bearer identifier is allocated to a Qos flow.A Non-GBR Qos flow is mapped to a default EPS bearer. A GBR Qos flow ismapped to a dedicated EPS bearer. EPS Qos information mapped by a Qosflow can be allocated by a PCC or an SMF. The EPS bearer identifiercorresponding to a QoS flow can be allocated by an SMF, an AMF or a UE.In order to support the handover between different systems, an SMF canhave functions of a PGW control plane. In a PCC deployment scenario, aPolicy Control Function (PCF) provides the SMF with mapped EPS Qos of aQos flow. In order to support the handover between different systems,the PCF can have a Policy Control and Charging Rules Function (PCRF).The SMF transmits, through the AMF and to the UE, the mapped EPS Qosinformation and/or an EPS bearer identifier of a Qos flow. For example,the SMF transmits the information to the UE through a non-access stratummessage PDU session establishment message. During the PDU sessionestablishment or GBR Qos flow establishment process, the SMF can furthertransmit, through the AMF and to an NG-RAN, the mapped EPS Qosinformation and/or an E-RAB identifier of a Qos flow. The AMF transmitsthe mapped EPS Qos information and/or an E-RAB identifier of a Qos flowto the NG-RAN through an initial context establishment request messageor a PDU session resource establishment request message. The NG-RAN cantransmit the mapped EPS Qos information and/or an E-RAB identifier of aQos flow to the UE through an RRC message. The E-RAB identifier and theEPS bearer identifier are identical or in one-to-one correspondence. Itis called an EPS bearer in a core network, while it is called an E-RABin an access network. The E-RAB identifier and the EPS bearer identifierare identical or in one-to-one correspondence. It is called the EPSbearer identifier in a core network, while it is called the E-RABidentifier in an access network.

The handover request further contains PDU session information to beestablished. The PDU session information to be established contains aPDU session identifier and information of a Qos flow or Qos flows in thePDU session. The handover request message contains the EPS beareridentifier and/or EPS Qos information mapped by a Qos flow in the PDUsession.

When the target NG-RAN node needs to initiate a handover of the UE to anEPS system, the target NG-RAN node uses the received EPS beareridentifier and/or EPS Qos information mapped by a Qos flow in the PDUsession so that the data forwarding during the handover betweendifferent systems can be supported. The specific process is as describedin the embodiment shown in FIG. 4 .

Step 603: The T-NG-RAN node transmits a handover response message to theS-NG-RAN node.

Step 604: The S-NG-RAN node transmits a handover command message to theUE. The UE is synchronized to the target base station.

Step 605: The UE transmits a handover completion message to the T-NG-RANnode.

Step 606: The T-NG-RAN node transmits a path switch request message toan AMF.

Step 607: The AMF transmits an N11 message to an SMF. This messagecontains the PDU session identifier. This message is specific to eachPDU session. The AMF transmits, through the N11 message, N2 sessioninformation received by the path switch request to the SMF.

Step 608: The SMF decides whether to hand over a UPF. For a PDU sessionto be handed over, the SMF selects a target UPF (T-UPF), and thenallocates the IP address of the T-UPF and the uplink/downlink tunnelidentifier. The SMF transmits an N4 session establishment requestmessage to the T-UPF. This message contains the address of the targetRAN and the uplink/downlink tunnel identifier.

Step 609: The T-UPF transmits an N4 session establishment responsemessage to the SMF.

Step 610: The SMF transmits an N4 session modification message to a PDUSession Anchor (PSA).

Step 611: The PSA transmits an N4 session modification response messageto the SMF. The PSA begins to transmit a downlink data packet to thetarget RAN through the T-UPF.

Step 612: The SMF transmits an N11 acknowledgement message to the AMF.This message contains the tunnel information allocated by the corenetwork.

Step 613: The AMF transmits a path switch request acknowledgementmessage to the T-NG-RAN node. Once the AMF has received the N11 downlinkresponses from all SMFs, the AMF transmits the collected core networktunnel information to the T-NG-RAN node.

Step 614: The T-NG-RAN node transmits a resource release message to theS-NG-RAN node.

So far, the fourth method for supporting handover according to thepresent invention has been described. By this method, the dataforwarding can be well supported when a UE accesses to an NG-RAN nodethrough a handover and when the NG-RAN node needs to initiate a handoverof the UE from a 5GS to an EPS, so that the loss of data is avoided, thedelay of the data forwarding is reduced and the continuity of servicesis ensured.

As shown in FIG. 7 , the method is used by an NG-RAN node to acquire theinformation for inter-system handover in order to support the scenariowhen a UE accesses to an NG-RAN node through an Xn handover procedureand the NG-RAN node needs to initiate a handover of the UE from a 5GS toan EPS. Here, the NG-RAN node can be a gNB or an eNB connected to the5GC (also called an ng-eNB). This method comprises the following steps.

Step 701: A source NG-RAN node (S-NG-RAN node) decides to initiate ahandover of a UE.

Step 702: The S-NG-RAN node transmits a handover request message to atarget NG-RAN node (T-NG-RAN node).

The UE has one or more ongoing PDU sessions. Each PDU session containsone or more Qos flows. During a PDU session establishment or GuaranteedBusiness Rate (GBR) Qos flow establishment process, mapped EPS Qosinformation and/or an EPS bearer identifier is allocated to a Qos flow.A Non-GBR Qos flow is mapped to a default EPS bearer. A GBR Qos flow ismapped to a dedicated EPS bearer. EPS Qos information mapped by a Qosflow can be allocated by a PCC or an SMF. The EPS bearer identifiercorresponding to a QoS flow can be allocated by an SMF, an AMF or a UE.In order to support the handover between different systems, an SMF canhave functions of a PGW control plane. In a PCC deployment scenario, aPolicy Control Function (PCF) provides the SMF with mapped EPS Qos of aQos flow. In order to support the handover between different systems,the PCF can have a Policy Control and Charging Rules Function (PCRF).The SMF transmits, through the AMF and to the UE, the mapped EPS Qosinformation and/or an EPS bearer identifier of a Qos flow. For example,the SMF transmits the information to the UE through a non-access stratummessage PDU session establishment message. During the PDU sessionestablishment or GBR Qos flow establishment process, the SMF can furthertransmit, through the AMF and to an NG-RAN, the mapped EPS Qosinformation and/or an E-RAB identifier of a Qos flow. The AMF transmitsthe mapped EPS Qos information and/or an E-RAB identifier of a Qos flowto the NG-RAN through an initial context establishment request messageor a PDU session resource establishment request message. The NG-RAN cantransmit the mapped EPS Qos information and/or an E-RAB identifier of aQos flow to the UE through an RRC message. The E-RAB identifier and theEPS bearer identifier are identical or in one-to-one correspondence. Itis called an EPS bearer in a core network, while it is called an E-RABin an access network. The E-RAB identifier and the EPS bearer identifierare identical or in one-to-one correspondence. It is called the EPSbearer identifier in a core network, while it is called the E-RABidentifier in an access network.

The handover request message contains a source-to-target transparenttransmitter. The handover request further contains PDU sessioninformation to be established. The PDU session information to beestablished contains a PDU session identifier.

703: The T-NG-RAN node transmits a handover response message to theS-NG-RAN node.

Step 704: The S-NG-RAN node transmits a handover command message to theUE. The UE is synchronized to the target base station.

Step 705: The UE transmits a handover completion message to the T-NG-RANnode.

Step 706: The T-NG-RAN node transmits a path switch request message toan AMF.

Step 707: The AMF transmits an N11 message to an SMF. This messagecontains the PDU session identifier. This message is specific to eachPDU session. The AMF transmits, through the N11 message, N2 sessioninformation received by the path switch request to the SMF.

Step 708: The SMF decides whether to hand over a UPF. For a PDU sessionto be handed over, the SMF selects a target UPF (T-UPF), and thenallocates the IP address of the T-UPF and the uplink/downlink tunnelidentifier. The SMF transmits an N4 session establishment requestmessage to the T-UPF. This message contains the address of the targetRAN and the uplink/downlink tunnel identifier.

Step 709: The T-UPF transmits an N4 session establishment responsemessage to the SMF.

Step 710: The SMF transmits an N4 session modification message to a PDUSession Anchor (PSA).

Step 711: The PSA transmits an N4 session modification response messageto the SMF. The PSA begins to transmit a downlink data packet to thetarget RAN through the T-UPF.

Step 712: The SMF transmits an N11 acknowledgement message to the AMF.This message contains the tunnel information allocated by the corenetwork. This message contains the EPS bearer identifier and/or EPS Qosinformation mapped by a Qos flow in the PDU session.

Step 713: The AMF transmits a path switch request acknowledgementmessage to the T-NG-RAN node. Once the AMF has received N11 downlinkresponses from all SMFs, the AMF gathers the received N2 sessioninformation and then transmits the gathered N2 session information tothe T-NG-RAN node. The N2 session information contains the core networktunnel identifier and the EPS bearer identifier and/or EPS Qosinformation mapped by a Qos flow in the PDU session. In this method ofthe present invention, the EPS bearer identifier can be allocated by theSMF, the AMF or the UE. If the EPS bearer identifier is allocated by theSMF, the EPS bearer identifier is contained in both steps 712 and 713.If the EPS bearer identifier is allocated by the UE, during the PDUsession establishment process, the SMF acquires, through the AMF andfrom the UE, the EPS bearer identifier mapped by a Qos flow, and the SMFtransmits the EPS bearer identifier to the target NG-RAN node in steps712 and 713. If the EPS bearer identifier is allocated by the AMF, theAMF transmits the EPS bearer identifier to the target NG-RAN node instep 713. Or, during the PDU session establishment process the AMFtransmits the allocated EPS bearer identifier mapped by the Qos flow tothe SMF; and during the handover process, the SMF transmits the EPSbearer identifier to the target NG-RAN node in steps 712 and 713.

When the target NG-RAN node needs to initiate a handover of the UE to anEPS system, the target NG-RAN node uses the received EPS beareridentifier and/or EPS Qos information mapped by the Qos flow in the PDUsession so that the data forwarding during the handover betweendifferent systems can be supported. The specific process is as describedin the embodiment shown in FIG. 4 .

Step 714: The T-NG-RAN node transmits a resource release message to theT-NG-RAN node.

So far, the fifth method for supporting handover according to thepresent invention has been described. By this method, the access of a UEto an NG-RAN node through a handover can be solved. When an NG-RAN nodeneeds to initiate a handover from a 5GS to an EPS, the data forwardingcan be well supported, so that the loss of data is avoided, the delay ofthe data forwarding is reduced and the continuity of services isensured.

As shown in FIG. 8 , the method is used for allocating, by a UE, an EPSbearer identifier corresponding to a Qos flow. When an NG-RAN node needsto initiate a handover of the UE to an EPS system, a method foracquiring information required by a handover between different systemsto support the handover is required. Here, the NG-RAN node can be a gNBor an eNB connected to the 5GC (also called an ng-eNB). Here, thedetailed description of steps irrelevant to the present invention, forexample, the processes and functions related to the Policy ControlFunction (PCF), the User Data management (UDM) and the Data Network(DN), will be omitted. This method comprises the following steps.

Step 801: A UE transmits a Non-Access Stratum (NAS) message to an AMF toinitiate a PDU session establishment process. To establish a new PDUsession, the UE generates a PDU session identifier. The NAS messagecontains the PDU session identifier and N1 Session Management (SM)information. The N1 SM information contains a PDU session establishmentrequest message.

An NG-RAN transmits the received NAS message to the AMF through an NGmessage. The NG message further contains the location information of auser and the type of the access technology.

Step 802: For a new PDU session to be established, the AMF selects anSMF. The AMF saves the PDU session identifier and the identifier of theSMF.

Step 803: The AMF transmits an SM request message to the SMF. Thismessage contains the N1 SM information received from the UE.

Step 804: The SMG triggers a PDU session authentication andauthorization process.

Step 805: If the PDU session establishment request is an initial requestand is not executed in step 804, the SMF transmits an N4 sessionestablishment message to the selected UPF. Otherwise, the SMF transmitsan N4 session modification request message to the selected UPF.

Step 806: The UPF transmits an N4 session establishment response messageor an N4 modification response message to the SMF.

Step 807: The SMF transmits an SM response message to the AMF. Thismessage contains N2 SM information and N1 SM information. The N1 SMinformation contains an NAS message PDU session establishment acception.The N2 SM information contains the PDU session identifier, Qos profile,core network tunnel information and S-NSSAI. The N2 SM informationcontains the EPS Qos information mapped by the Qos flow in the PDUsession. The N1 SM information contains the EPS Qos information mappedby the Qos flow in the PDU session.

Step 808: The AMF transmits a PDU session resource establishment requestmessage to an NG-RAN. The message contains the N2 SM information and theNAS message PDU session establishment acception. The NG-RAN saves thePDU session information, information of the Qos flow in the DPU session,and/or the EPS Qos information mapped by the Qos flow.

Step 809: The NG-RAN transmits an access network resource establishmentrequest message to the UE. The access network resource establishmentrequest message can be an RRC connection reconfiguration message. TheNG-RAN allocates NG interface user plane downlink tunnel information.The message contains the NAS message PDU session establishment acceptionmessage. If the NG-RAN can allocate the required resources and thetunnel information of the NG-RAN side, the NG-RAN transmits an NASmessage to the UE. The RRC message transmitted to the UE by the NG-RANor the NAS message contained in the RRC message contains the EPS Qosflow information mapped by the Qos flow.

Step 810: The UE transmits an access network resource establishmentmessage to the NG-RAN. The access network resource establishment messagecan be an RRC connection reconfiguration completion message. The UEallocates the EPS bearer identifier corresponding to the Qos flow. TheUE allocates, according to the Qos of the Qos flow and the mapped EPSQos information, the EPS bearer identifier corresponding to the Qosflow. This message contains the EPS bearer identifier mapped by a Qosflow. This message can further contain an NAS message PDU sessionestablishment completion message. The NAS message contains the EPSbearer identifier mapped by a Qos flow.

Step 811: The NG-RAN saves the EPS bearer identifier mapped by a Qosflow received from the UE. The NG-RAN transmits a PDU session resourceestablishment response message to the AMF. This message contains the N2SM information and the PDU session identifier. The N2 SM informationcontains the PDU session identifier, RAN tunnel information, theaccepted Qos information and/or the rejected Qos flow information. Thismessage or the N2 SM information further contains the EPS beareridentifier mapped by a Qos flow. This message or the N2 SM informationfurther contains the NAS message PDU session establishment completionmessage.

When the NG-RAN needs to initiate a handover of the UE to an EPS system,the NG-RAN uses the EPS bearer identifier and/or EPS Qos informationmapped by a Qos flow in the PDU session so that the data forwardingduring the handover between different systems can be supported. Thespecific process is as described in the embodiment shown in FIG. 4 .

Step 812: The AMF transmits an SM request message to the SMF. Thismessage contains the N2 SM information received from the NG-RAN. Thismessage contains the NAS message received from the UE. This messagecontains the EPS bearer identifier mapped by a Qos flow.

Step 813: If the N4 session of the corresponding PDU session has notbeen established, the SMF initiates an N4 session establishment processto the UPF. Otherwise, the SMF initiates an N4 session modificationprocess. The SMF transmits the tunnel information allocated by theaccess network and/or the core network tunnel information to the UPF.

Step 814: The UPF transmits an N4 session modification response messageor an N4 session request response message to the SMF.

Step 815: The SMF transmits an SM response message to the AMF.

So far, the sixth method for supporting handover according to thepresent invention has been described. By this method, the allocation ofEPS bearer identifier by the UE can be solved. When an NG-RAN needs toinitiate a handover of the UE from the 5GS to the EPS, the dataforwarding can be well supported, so that the loss of data is avoided,the delay of data forwarding is reduced and the continuity of servicesis ensured.

As shown in FIG. 9 , the method is used for allocating, by a UE, an EPSbearer identifier corresponding to a Qos flow. When an NG-RAN node needsto initiate a handover of the UE to an EPS system, a method foracquiring information required by a handover between different systemsto support the handover is required. Here, the NG-RAN node can be a gNBor an eNB connected to the 5GC (also called an ng-eNB). Here, thedetailed description of steps irrelevant to the present invention, forexample, the functions related to the Policy Control Function (PCF), theUser Data management (UDM) and the Data Network (DN), will be omitted.This method comprises the following steps.

Steps 901 to 909 are the same as steps 801 to 809 and will not berepeated here.

Step 910: The UE transmits an access network resource establishmentmessage to the NG-RAN. The access network resource establishment messagecan be an RRC connection reconfiguration completion message. The UEallocates the EPS bearer identifier corresponding to the Qos flow. TheUE allocates, according to the Qos of the Qos flow and the mapped EPSQos information, the EPS bearer identifier corresponding to the Qosflow. This message can further contain an NAS message PDU sessionestablishment completion message. The NAS message contains the EPSbearer identifier mapped by a Qos flow.

Step 911: The NG-RAN transmits a PDU session resource establishmentresponse message to the AMF. This message contains the N2 SM informationand the PDU session identifier. The N2 SM information contains the PDUsession identifier, RAN tunnel information, the accepted Qos informationand/or the rejected Qos flow information. This message or the N2 SMinformation further contains the NAS message PDU session establishmentcompletion message.

Step 912: The AMF transmits an SM request message to the SMF. Thismessage contains the N2 SM information received from the NG-RAN. Thismessage contains the NAS message received from the UE. The NAS messagecontains the EPS bearer identifier mapped by a Qos flow.

Step 913: If the N4 session of the corresponding PDU session has notbeen established, the SMF initiates an N4 session establishment processto the UPF. Otherwise, the SMF initiates an N4 session modificationprocess. The SMF transmits the tunnel information allocated by theaccess network and/or the core network tunnel information to the UPF.

Step 914: The UPF transmits an N4 session modification response messageor an N4 session request response message to the SMF.

Step 915: The SMF transmits an SM response message to the AMF. Thismessage contains the N2 SM information. This message contains the EPSbearer identifier mapped by a Qos flow received from the UE. Thismessage can further contain the EPS Qos information mapped by the Qosflow.

Step 916: The AMF transmits a PDU session resource establishmentcompletion message to the NG-RAN. This message contains the EPS beareridentifier mapped by a Qos flow. This message can further contain theEPS Qos information mapped by the Qos flow.

When the NG-RAN needs to initiate a handover of the UE to an EPS system,the NG-RAN uses the EPS bearer identifier and/or EPS Qos informationmapped by a Qos flow in the PDU session so that the data forwardingduring the handover between different systems can be supported. Thespecific process is as described in the embodiment shown in FIG. 4 .

So far, the seventh method for supporting handover according to thepresent invention has been described. By this method, the allocation ofan EPS bearer identifier by the UE can be solved. When an NG-RAN needsto initiate a handover of the UE from the 5GS to the EPS, the dataforwarding can be well supported, so that the loss of data is avoided,the delay of data forwarding is reduced and the continuity of servicesis ensured.

As shown in FIG. 10 , the method is a method for supporting dataforwarding when a PDU session of a UE is to be handed over from the 5GCto the EPC in the case of dual registration. This method comprises thefollowing steps.

Step 1001: A UE transmits a PDU connection request message to an MME.This message contains a handover indication.

Step 1002: The MME receives the PDU connection request message. If thereceived message contains the handover indication information, the MMEuses a PDN GW that is obtained from the subscription data in a locationupdate process in an adhesion process. If the request is an initialrequest, the MME selects a PDN GW according to the PDN GW selectionfunction (referring to TS23. 401 4.3.8.1). The MME saves the receivedhandover indication.

The MME transmits a session establishment request message to an SGW.This message contains a PDN GW address. This message contains a handoverindication.

Step 1003 a: The SGW transmits a session establishment request messageto a PDN GW (PGW). The SGW transmits this message to the PGW indicatedby the PDN GW address received in step 1002. This message contains ahandover indication. The SGW saves the received handover indication. Ifthe SGW receives the handover indication from the MME, the SGW containsthe handover indication in the session establishment request message tobe transmitted to the PGW.

The process with the PCRF is not the focus of the present invention andthus will not be described in detail.

Step 1003 b: The PGW transmits a session establishment response messageto the SGW. If there is the handover indication, the PGW does nottransmit downlink data to the SGW, and instead waits for the downlinkpath switch in step 1012 a. If there is the handover indication, the PGWdetermines, according to the Qos of the service, whether the PDNconnection requires data forwarding. The PGW can determine the dataforwarding for the PDN connection or whether the data forwarding isrequired for each EPS bearer in the PDN connection. If the dataforwarding is required, the PGW contains a data forwarding indication inthe session establishment response message. The data forwardingindication can be specific to the PDU connection or the absence of theEPS bearer.

Step 1004: The SGW transmits a session establishment response message tothe MME. If the UE has indicated the handover, the successfulestablishment of a bearer over an S5/S8 interface can be informed to theMME by this message. If the SGW has received the data forwardingindication from the PGW, the SGW contains this indication in the sessionestablishment response message to be transmitted to the MME.

Step 1005: The MME transmits a bearer establishment request message toan eNB. This message contains the handover indication or the dataforwarding indication. If the MME has received the handover indicationfrom the UE and/or the MME has received the data forwarding indicationfrom the SGW, the MME contains the handover indication or the dataforwarding indication in the bearer establishment request message. Thedata forwarding indication can be specific to all E-RABs or specific toeach E-RAB. The bearer establishment request message can also be aninitial context establishment request message.

Step 1006: The eNB transmits an RRC connection reconfiguration messageto the UE.

Step 1007: The UE transmits an RRC connection reconfiguration completionmessage to the eNB.

Step 1008: The eNB transmits a bearer establishment response message tothe MME. If the eNB has received the data forwarding indicationinformation, the eNB allocates tunnel information used for downlink dataforwarding to an E-RAB. The tunnel information contains a tunnelidentifier and the address of a transport layer. The eNB contains thedata forwarding tunnel information in the bearer establishment responsemessage.

Step 1009: The UE transmits a direct transfer message to the eNB. Thismessage contains an NAS message, i.e., a PDN connection completionmessage.

Step 1010: The eNB transmits the received PDU connection completionmessage to the MME.

Step 1011: The MME transmits a bearer modification request message tothe SGW. This message contains the data forwarding tunnel informationreceived from the eNB. This message contains a handover indication. TheSGW allocates the tunnel information for downlink data forwarding. Thetunnel information contains a tunnel identifier and an address of atransport layer. The tunnel information is specific to each EPS bearer.

Step 1012: The SGW transmits a bearer modification request message tothe PGW. This message contains the data forwarding tunnel informationreceived from the MME or the downlink data forwarding tunnel informationallocated by the SGW in step 1011. If this message contains the handoverindication, the PGW transmits the data of the corresponding PDNconnection to the SGW.

If the PGW has received the data forwarding tunnel information,corresponding to the PDN connection transferred from the 5GC to the EPC,the PGW allocates uplink tunnel information for data forwarding betweenthe NG-RAN and the UPF. In order to support the movement betweendifferent systems, the PGW further has a function of an SMF controlplane. Or, the PGW informs the UPF of the uplink tunnel information fordata forwarding between the NG-RAN and the UPF. The PGW transmits, tothe UPF, the EPS bearer identifier mapped by a Qos flow in the PDUsession. The PGW transmits, through the AMF and to the NG-RAN, theuplink tunnel information for data forwarding between the NG-RAN and theUPF. The uplink tunnel information for data forwarding between theNG-RAN and the UPF is specific to each PDU session.

If the NG-RAN has received the data forwarding uplink tunnelinformation, the NG-RAN forwards downlink data to the UPF. The UPFtransmits, according to the mapping between the Qos flow in the PDUsession and the EPS bearer identifier and through a tunnel for themapped EPS bearer, data of different Qos flows in the PDU session to theSGW or the PGW. The PGW transmits the received forwarded data to theSGW. The SGW transmits the received forwarded data to the eNB. Or, thePGW directly transmits the received forwarded data to the eNB.

The eNB transmits the forwarded data to the UE, and then transmits otherdata.

Step 1013: The PGW transmits a bearer modification response message tothe SGW.

Step 1014: The SGW transmits the bearer modification response message tothe MME. The SGW begins to transmit downlink cached data.

So far, the eighth method for supporting handover according to thepresent invention has been described. By this method, during thetransfer of a PDU session of a dual-connection UE from the 5GC to theEPC, the data forwarding can be well supported, so that the loss of datais avoided, the delay of data forwarding is reduced and the continuityof services is ensured.

The present invention provides a method for supporting handover,comprising the following steps of:

acquiring, by a first base station in a first communication system, Qosflow information corresponding to EPS bearer information through anintra-system handover process;

transmitting, by the first base station, a handover required message toa Mobility Management Entity (MME), wherein the handover requiredmessage contains the Qos flow information corresponding to EPS bearerinformation;

receiving, by the first base station, a handover command messagecarrying data forwarding tunnel information transmitted by the MME; and

performing, by the first base station, data forwarding according to thereceived data forwarding tunnel information.

The Qos flow information corresponding to EPS bearer informationacquired by the first base station contains Qos and/or a Qos flowidentifier.

the handover required message contains the PDU session identifier and adownlink data forwarding proposal for the PDU session. The firstcommunication system is an LTE communication system.

The method will be further described by the following specificembodiments.

As shown in FIG. 11 , this method is used for supporting data forwardingduring the handover of a UE from the EPC to the 5GC. This methodcomprises the following steps.

Step 1101: A UE transmits a PDU connection request message to an MME.The UE allocates a PDU session identifier and then transmits the PDUsession identifier to the MME.

Step 1102: The MME receives the PDU connection request message.

The MME transmits a session establishment request message to an SGW.This message contains a PDN GW address. This message contains the PDUsession identifier.

Step 1103 a: The SGW transmits a session establishment request messageto a PDN GW (PGW). The SGW transmits this message to the PGW indicatedby the PDN GW address received in step 1002. This message contains thePDU session identifier.

The process with the PCRF is not the focus of the present invention andthus will not be described in detail.

Step 1103 b: The PGW transmits a session establishment response messageto the SGW. In order to support the handover between different systems,the PGW further has a function of an SMF. The PDU session to which theEPS bearer belongs can be allocated by the function of the PCC or PGWcontrol plane. The Qos information and/or Qos flow identifier of amapped Qos flow of an EPS bearer can be determined by the PCC or PGW. Ina PCC deployment scenario, the Policy Control and Charging RulesFunction (PCRF) provides the SMF with the identifier of the PDU sessionto which the EPS bearer belongs, and the PCRF provides the SMF with theQos and/or Qos flow identifier of a Qos flow mapped by the EPS bearer.Default EPS bearers are mapped to a non-Guaranteed Business Rate(non-GBR) Qos flows. The PGW transmits, to the SGW, the identifier ofthe PDU session to which the EPS bearer belongs, the Qos information ofa Qos flow mapped by the EPS bearer and/or the identifier of a Qos flowmapped by the EPS bearer. The SGW saves the received identifier of thePDU session to which the EPS bearer belongs. The SGW saves the receivedQos and/or Qos flow identifier of a Qos flow mapped by the EPS bearer.

Step 1104: The SGW transmits a session establishment response message tothe MME. This message contains the identifier of the PDU session towhich the EPS bearer belongs. This message contains the Qos informationand/or Qos flow identifier of a mapped Qos flow of the EPS bearer. TheMME saves the identifier of the PDU session to which the EPS bearerbelongs. The MME saves the received Qos and/or Qos flow identifier of aQos flow mapped by the EPS bearer.

Step 1105: The MME transmits a bearer establishment request message toan eNB. The message contains an NAS message, i.e., a PDU connectionestablishment acception message. This message contains the identifier ofthe PDU session to which the EPS bearer belongs. This message containsthe Qos information and/or Qos flow identifier of a mapped Qos flow theEPS bearer. The bearer establishment request message can also be aninitial context establishment request message.

When the eNB needs to initiate a handover of the UE to the NG-RAN, theeNB gives a proposal of downlink data forwarding. The eNB can give aproposal of downlink data forwarding for each PDU session or for eachQos flow. The eNB determines, according to the information about the PDUsession to which the EPS bearer and/or the Qos flow mapped by the EPSbearer received from the MME, to give a proposal of downlink dataforwarding. The eNB contains the PDU session identifier and the downlinkdata forwarding in a source-to-target transparent transmitter. Or, theeNB contains the PDU session identifier, the Qos flow identifier in thePDU session and the downlink data forwarding information in asource-to-target transparent transmitter. If the target base station hasaccepted the data forwarding, the source base station can forward datato the SGW according to a tunnel corresponding to each EPS bearer or atunnel corresponding to each PDU session. The specific process is asdescribed in the embodiment shown in FIG. 14 .

Step 1106: The eNB transmits an RRC connection reconfiguration messageto the UE. The message contains an NAS message, i.e., a PDU connectionestablishment acception message. This message or the NAS PDU connectionestablishment acception message contains the Qos information and/or Qosflow identifier of a mapped Qos flow of the EPS bearer and/or theidentifier of the PDU session to which the EPS bearer belongs.

Step 1107: The UE transmits an RRC connection reconfiguration completionmessage to the eNB. The UE saves the Qos information and/or Qos flowidentifier of a mapped Qos flow of the EPS bearer and/or the identifierof the PDU session to which the EPS bearer belongs.

Step 1108: The eNB transmits a bearer establishment response message tothe MME.

Step 1109: The UE transmits a direct transfer message to the eNB. Thismessage contains an NAS message, i.e., a PDN connection completionmessage.

Step 1110: The eNB transmits the received PDU connection completionmessage to the MME.

Step 1111: The MME transmits a bearer modification request message tothe SGW.

Step 1112: The SGW transmits a bearer modification request message tothe PGW.

Step 1113: The PGW transmits a bearer modification response message tothe SGW.

Step 1114: The SGW transmits the bearer modification response message tothe MME.

So far, the ninth method for supporting handover according to thepresent invention has been described. By this method, the informationabout a Qos flow mapped by an EPS bearer can be obtained by the eNB inadvance. Therefore, when a UE is handed over from the EPS to the 5GS,the data forwarding can be well supported, so that the loss of data isavoided, the delay of data forwarding is reduced and the continuity ofservices is ensured.

As shown in FIG. 12 , the method is used for accessing a UE to an eNBthrough an S1 handover process. When the eNB needs to initiate ahandover of the UE to a 5GS, a method for acquiring information requiredby a handover between different systems to support the handover isrequired. This method comprises the following steps.

Step 1201: A source eNB (S-eNB) decides to initiate a handover of a UE.

Step 1202: The S-eNB transmits a handover required message to an MME.

The UE has one or more ongoing PDU sessions (also referred to as PDNconnections). Each PDU session contains one or more EPS bearers. Duringa PDU session establishment or EPS bearer establishment process, Qosinformation and/or Qos flow identifier of the mapped Qos flow and/or theidentifier of the PDU session to which the EPS bearer belongs areallocated to each EPS bearer. Default EPS bearers are mapped to anon-Guaranteed Business Rate (non-GBR) Qos flow. The PDU session towhich the EPS bearer belongs can be allocated by the function of the PCCor PGW control plane. The Qos information and/or Qos flow identifier ofthe mapped Qos flow of the EPS bearer can be allocated by a function ofa PCC or PGW control plane. In order to support the handover betweendifferent systems, a PGW control plane can further have an SMF function.In a PCC deployment scenario, the Policy Control and Charging RulesFunction (PCRF) provides the SMF with the identifier of the PDU sessionto which the EPS bearer belongs, and the PCRF provides the SMF with theQos and/or Qos flow identifier of the Qos flow mapped by the EPS bearer.In order to support the handover between different systems, a PCRFfurther has a Policy Control Function (PCF). The PGW transmits, to theSGW, the identifier of the PDU session to which the EPS bearer belongs,the Qos information of the Qos flow mapped by the EPS bearer and/or theidentifier of the Qos flow mapped by the EPS bearer. The SGW saves thereceived identifier of the PDU session to which the EPS bearer belongs.The SGW saves the received Qos and/or Qos flow identifier of the Qosflow mapped by the EPS bearer. The SMF transmits, through the MME and tothe UE, the Qos and/or Qos flow identifier of the Qos flow mapped by theEPS bearer and/or the identifier of the PDU session to which the EPSbearer belongs. For example, the SMF transmits the information to the UEthrough a non-access stratum message, i.e., a PDN connectionestablishment message. During a PDU session establishment or EPS bearerestablishment process, the MME transmits, to an E-UTRAN, the Qos and/orQos flow identifier of the Qos flow mapped by the EPS bearer and/or theidentifier of the PDU session to which the EPS bearer belongs. The MMEacquires, from the SMF, the Qos and/or Qos flow identifier of the Qosflow mapped by the EPS bearer and/or the identifier of the PDU sessionto which the EPS bearer belongs. The E-UTRAN can transmit the mappinginformation to the UE through an RRC message. The E-RAB identifier andthe EPS bearer identifier are identical or in one-to-one correspondence.It is called an EPS bearer in a core network, while it is called anE-RAB in an access network. The E-RAB identifier and the EPS beareridentifier are identical or in one-to-one correspondence. It is calledthe EPS bearer identifier in a core network, while it is called theE-RAB identifier in an access network.

In the handover required message or a source-to-target transparenttransmitter, the S-eNB contains the identifier of the PDU session towhich the EPS bearer belongs, the Qos information of the Qos flow mappedby the EPS bearer and/or the identifier of the Qos flow mapped by theEPS bearer.

Step 1203: The MME transmits a handover request message to a target eNB(T-eNB). This message contains a source-to-target transparenttransmitter. The handover request message or the source-to-targettransparent transmitter contains the identifier of the PDU session towhich the EPS bearer belongs, the Qos information of the Qos flow mappedby the EPS bearer and/or the identifier of the Qos flow mapped by theEPS bearer. The handover request further contains information about anE-RAB to be established. The information about an E-RAB to beestablished contains an E-RAB identifier, E-RAB Qos information, uplinktunnel information or more. In this method of the present invention,there are two methods for transmitting, to the T-eNB, the identifier ofthe PDU session to which the EPS bearer belongs, the Qos information ofthe Qos flow mapped by the EPS bearer and/or the identifier of the Qosflow mapped by the EPS bearer.

Way 1: A source-to-target transparent transmitter is used, as describedin step 1202 and in this step.

Way 2: The MME transmits, to the T-eNB and through a handover requestmessage, the identifier of the PDU session to which the EPS bearerbelongs, the Qos information of the Qos flow mapped by the EPS bearerand/or the identifier of the Qos flow mapped by the EPS bearer. The MMEhas received, from the SGW, and saved the identifier of the PDU sessionto which the EPS bearer belongs, the Qos information of the Qos flowmapped by the EPS bearer and/or the identifier of the Qos flow mapped bythe EPS bearer by the method shown in FIG. 11 .

When the target eNB needs to initiate a handover of the UE to a 5GS, thetarget eNB uses the received identifier of the PDU session to which theEPS bearer belongs, the Qos information of the Qos flow mapped by theEPS bearer and/or the identifier of the Qos flow mapped by the EPSbearer so that the data forwarding during the handover between differentsystems can be supported. The specific process is as described in theembodiment shown in FIG. 14 .

Step 1204: The eNB transmits a handover request acknowledgement messageto the MME. This message contains the data forwarding tunnel informationallocated by the target base station. This message contains atarget-to-source transparent transmitter.

Step 1205: The MME transmits a handover command message to the S-eNB.This message contains a target-to-source transparent transmitter.

Step 1206: The S-eNB transmits a handover command message to the UE. TheUE is synchronized to the target base station.

Step 1207: The UE transmits a handover completion message to the T-eNB.

Step 1208: The eNB transmits a handover notification message to the MME.

Step 1209: The MME transmits a bearer modification request message tothe SGW. This message is specific to each PDU session or PDU connection.

Step 1210: The SGW transmits a bearer modification response message tothe MME.

Step 1211: The MME transmits a UE context release command message to theS-eNB.

Step 1212: The S-eNB transmits a UE context release completion messageto the MME.

So far, the tenth method for supporting handover according to thepresent invention has been described. By this method, the access of a UEto an eNB through a handover can be solved. When an eNB needs toinitiate a handover from an EPS to a 5GS, the data forwarding can bewell supported, so that the loss of data is avoided, the delay of dataforwarding is reduced and the continuity of services is ensured.

As shown in FIG. 13 , this method is used for accessing a UE to an eNBthrough an X2 handover process. When the eNB needs to initiate ahandover of the UE to a 5GS, a method for acquiring information requiredby a handover between different systems to support the handover isrequired. This method comprises the following steps.

Step 1301: A source eNB (S-eNB) decides to initiate a handover of a UE.

Step 1302: The S-eNB transmits a handover request message to a targeteNB (T-eNB).

The UE has one or more ongoing PDU sessions (also referred to as PDNconnections). Each PDU session contains one or more EPS bearers. Duringa PDU session establishment or EPS bearer establishment process, Qosinformation and/or Qos flow identifier of the mapped Qos flow and/or theidentifier of a PDU session are allocated to each EPS bearer. DefaultEPS bearers are mapped to a non-Guaranteed Business Rate (non-GBR) Qosflow. The PDU session to which the EPS bearer belongs can be allocatedby the function of the PCC or PGW control plane. The Qos informationand/or Qos flow identifier of the mapped Qos flow of the EPS bearer canbe allocated by a function of a PCC or PGW control plane. In order tosupport the handover between different systems, a PGW control plane canfurther have an SMF function. In a PCC deployment scenario, the PolicyControl and Charging Rules Function (PCRF) provides the SMF with theidentifier of the PDU session to which the EPS bearer belongs, and thePCRF provides the SMF with the Qos and/or Qos flow identifier of the Qosflow mapped by the EPS bearer. In order to support the handover betweendifferent systems, a PCRF further has a Policy Control Function (PCF).The PGW transmits, to the SGW, the identifier of the PDU session towhich the EPS bearer belongs, the Qos information of the Qos flow mappedby the EPS bearer and/or the identifier of the Qos flow mapped by theEPS bearer. The SGW saves the received identifier of the PDU session towhich the EPS bearer belongs. The SGW saves the received Qos and/or Qosflow identifier of the Qos flow mapped by the EPS bearer. The SMFtransmits, through the MME and to the UE, the Qos and/or Qos flowidentifier of the Qos flow mapped by the EPS bearer and/or theidentifier of the PDU session to which the EPS bearer belongs. Forexample, the SMF transmits the information to the UE through anon-access stratum message, i.e., a PDN connection establishmentmessage. During a PDU session establishment or EPS bearer establishmentprocess, the MME transmits, to an E-UTRAN, the Qos and/or Qos flowidentifier of the Qos flow mapped by the EPS bearer and/or theidentifier of the PDU session to which the EPS bearer belongs. The MMEacquires, from the SMF, the Qos and/or Qos flow identifier of the Qosflow mapped by the EPS bearer and/or the identifier of the PDU sessionto which the EPS bearer belongs. The E-UTRAN can transmit the mappinginformation to the UE through an RRC message. The E-RAB identifier andthe EPS bearer identifier are identical or in one-to-one correspondence.It is called an EPS bearer in a core network, while it is called anE-RAB in an access network. The E-RAB identifier and the EPS beareridentifier are identical or in one-to-one correspondence. It is calledthe EPS bearer identifier in a core network, while it is called theE-RAB identifier in an access network.

The handover request further contains information about an E-RAB to beestablished. The information about an E-RAB to be established containsan E-RAB identifier, E-RAB Qos information, uplink tunnel information ormore.

In the handover request message or a source-to-target transparenttransmitter, the S-eNB contains the identifier of the PDU session towhich the EPS bearer belongs, the Qos information of the Qos flow mappedby the EPS bearer and/or the identifier of the Qos flow mapped by theEPS bearer.

In this method of the present invention, there are three methods forinforming the target eNB of the identifier of the PDU session to whichthe EPS bearer belongs, the Qos information of the Qos flow mapped bythe EPS bearer and/or the identifier of the Qos flow mapped by the EPSbearer.

Method 1: The target eNB is informed by the source eNB through ahandover request message, as described in this step.

Method 2: The target base station is informed by the MME by step 1309,as described in step 1309.

Method 3: The target eNB is informed by the SGW through the MME by steps1308 and 1309, as described in steps 1308 and 1309.

When the target eNB needs to initiate a handover of the UE to a 5GS, thetarget eNB uses the received identifier of the PDU session to which theEPS bearer belongs, the Qos information of the Qos flow mapped by theEPS bearer and/or the identifier of the Qos flow mapped by the EPSbearer so that the data forwarding during the handover between differentsystems can be supported. The specific process is as described in theembodiment shown in FIG. 14 .

Step 1303: The T-eNB transmits a handover response message to the S-eNB.

Step 1304: The S-eNB transmits a handover command message to the UE. TheUE is synchronized to the target base station.

Step 1305: The UE transmits a handover completion message to the T-eNB.

Step 1306: The T-eNB transmits a path switch request message to the MME.

Step 1307: The MME transmits a bearer modification request message tothe SGW.

Step 1308: The SGW transmits a bearer modification response message tothe MME. This message contains the identifier of the PDU session towhich the EPS bearer belongs, the Qos information of the Qos flow mappedby the EPS bearer and/or the identifier of the Qos flow mapped by theEPS bearer.

Step 1309: The MME transmits a path switch request acknowledgementmessage to the T-eNB. This message contains the identifier of the PDUsession to which the EPS bearer belongs, the Qos information of the Qosflow mapped by the EPS bearer and/or the identifier of the Qos flowmapped by the EPS bearer.

When the target eNB needs to initiate a handover of the UE to a 5GS, thetarget eNB uses the received identifier of the PDU session to which theEPS bearer belongs, the Qos information of the Qos flow mapped by theEPS bearer and/or the identifier of the Qos flow mapped by the EPSbearer so that the data forwarding during the handover between differentsystems can be supported. The specific process is as described in theembodiment shown in FIG. 14 .

Step 1310: The T-eNB transmits a resource release message to the S-eNB.

So far, the eleventh method for supporting handover according to thepresent invention has been described. By this method, the access of a UEto an eNB through a handover can be solved. When an eNB needs toinitiate a handover from an EPS to a 5GS, the data forwarding can bewell supported, so that the loss of data is avoided, the delay of dataforwarding is reduced and the continuity of services is ensured.

FIG. 14 shows a schematic flowchart of a first method for handing over aUE from an EPS to a 5G system in the present invention. In this method,the information is obtained by the methods for supporting handover shownin FIGS. 11, 12 and 13 , and the data forwarding during the handoverbetween different systems can thus be well supported. This methodcomprises the following steps.

Step 1401: An E-UTRAN decides to hand over a UE to an NG-RAN.

Here, the E-UTRAN can be an eNB connected to the EPC. The NG-RAN can bea gNB, an eNB connected to the 5GC or a Centralized Unit (CU) in thegNB. The eNB connected to the 5GC can also be referred to as an ng-eNB.

The user plane path before handover is anchors UPF, SGW and E-UTRAN. TheSGW needs to support an interface to the anchor UPF. The anchor UPF canbe located in the 5GC or the EPC or can be a common entity. The anchorUPF can be a UPF serving the UE, and executes a function of a user planeanchor during the handover between different Radio Access Technologies(RATs). The anchor UPF can be a function of a PGW user plane plus an UPFor a function of an UPF plus a PGW user plane, and executes a functionof a user plane anchor during the handover between different RadioAccess Technologies (RATs).

The UE has one or more ongoing PDU sessions (also referred to as PDNconnections). Each PDU session contains one or more EPS bearers. Duringa PDU session establishment or EPS bearer establishment process, Qosinformation and/or Qos flow identifier of the mapped Qos flow and/or theidentifier of a PDU session are allocated to each EPS bearer. DefaultEPS bearers are mapped to a non-Guaranteed Business Rate (non-GBR) Qosflow. The PDU session to which the EPS bearer belongs can be allocatedby the function of the PCC or PGW control plane. The Qos informationand/or Qos flow identifier of the mapped Qos flow of the EPS bearer canbe allocated by a function of a PCC or PGW control plane. In order tosupport the handover between different systems, a PGW control plane canfurther have an SMF function. In a PCC deployment scenario, the PolicyControl and Charging Rules Function (PCRF) provides the SMF with theidentifier of the PDU session to which the EPS bearer belongs, and thePCRF provides the SMF with the Qos and/or Qos flow identifier of the Qosflow mapped by the EPS bearer. In order to support the handover betweendifferent systems, a PCRF further has a Policy Control Function (PCF).The PGW transmits, to the SGW, the identifier of the PDU session towhich the EPS bearer belongs, the Qos information of the Qos flow mappedby the EPS bearer and/or the identifier of the Qos flow mapped by theEPS bearer. The SGW saves the received identifier of the PDU session towhich the EPS bearer belongs. The SGW saves the received Qos and/or Qosflow identifier of the Qos flow mapped by the EPS bearer. The SMFtransmits, through the MME and to the UE, the Qos and/or Qos flowidentifier of the Qos flow mapped by the EPS bearer and/or theidentifier of the PDU session to which the EPS bearer belongs. Forexample, the SMF transmits the information to the UE through anon-access stratum message, i.e., a PDN connection establishmentmessage. During a PDU session establishment or EPS bearer establishmentprocess, the MME transmits, to an E-UTRAN, the Qos and/or Qos flowidentifier of the Qos flow mapped by the EPS bearer and/or theidentifier of the PDU session to which the EPS bearer belongs. The MMEacquires, from the SMF, the Qos and/or Qos flow identifier of the Qosflow mapped by the EPS bearer and/or the identifier of the PDU sessionto which the EPS bearer belongs. The E-UTRAN can transmit the mappinginformation to the UE through an RRC message. The E-RAB identifier andthe EPS bearer identifier are identical or in one-to-one correspondence.It is called an EPS bearer in a core network, while it is called anE-RAB in an access network. The E-RAB identifier and the EPS beareridentifier are identical or in one-to-one correspondence. It is calledthe EPS bearer identifier in a core network, while it is called theE-RAB identifier in an access network.

Step 1402: The E-UTRAN transmits a handover required message to an MME.This message contains the identifier of a target NG-RAN node and asource-to-target transparent transmitter. This message further containsidentification information indicating an AMF to which the target NG-RANis connected. The identification information can be an identifier of atracked area, an identifier of a network slice, an identifier of an AMFpool, an identifier of the AMF or more.

This message contains the handover type. The handover type contains ahandover in the LTE, a handover from the LTE to the NR or more. Thehandover from the LTE to the NR can further contain a handover from anLTE base station connected to the EPC to the NR or a handover from anLTE base station connected to the 5GC to the NR. This is because an LTEeNB may support an interface to the 5GC, or may not support an interfaceto the 5GC.

In order to support the handover between different systems, an SMF canhave functions of a PGW control plane.

The source-to-target transparent transmitter contains a PDU sessionidentifier and a downlink data forwarding proposal for the PDU session.Or, the source-to-target transparent transmitter contains the PDUsession identifier, the identifier of a Qos flow in the PDU session anda downlink data forwarding proposal for the Qos flow. The eNB obtains,through a PDN connection establishment process or a handover process,the identifier of the PDU session corresponding to the EPS bearer and/orthe identifier of the Qos flow in the PDU session. It is also possibleto obtain the Qos information of the mapped Qos flow. The method forobtaining, by the eNB, the information through a handover is asdescribed in the methods shown in FIGS. 12 and 13 . The method forobtaining, by the eNB, the information about the mapping from the E-RABto the PDU session and/or from the E-RAB to the Qos flow in the PDUsession through a PDU session establishment process is as described inFIG. 11 . The eNB decides, according to the mapping from the E-RAB tothe Qos flow in the PDU session and the Qos information, whether to givea proposal of data forwarding. The eNB can make a decision byconsidering other factors, for example, the presence or absence of datain a buffer, without influencing the main contents of the presentinvention.

The handover required message can further contain PDU sessioninformation and information about the Qos flow in the PDU session. ThePDU session information contains a PDU session identifier. Theinformation about the Qos flow contains a Qos flow identifier and/or Qosinformation of the Qos flow. The E-UTRAN obtains, through a PDNconnection establishment process or a handover process, the identifierof the PDU session to which the EPS bearer belongs, the identifier ofthe mapped Qos flow and/or the Qos information of the mapped Qos flow.The process of obtaining, by the E-UTRAN, the information through thePDN connection establishment process is as described in FIG. 11 and step1401. The method for obtaining, by the E-UTRAN, the information throughthe handover process is as described in the FIGS. 12 and 13 .

Step 1403: The MME transmits a relocation request message to an AMF.According to the identification information indicating the AMF connectedto the target NG-RAN node contained in the handover required message,the MME selects and finds an AMF. This message contains the identifierof the target NG-RAN node, a source-to-target transparent transmitterand UE context information. The UE context information contains UEMobility Management (MM) context information and Session Management (SM)context. This message contains the handover type. The handover typecontains a handover in the LTE, a handover from the LTE to the NR ormore. The handover from the LTE to the NR can further contain a handoverfrom an LTE base station connected to the EPC to the NR or a handoverfrom an LTE base station connected to the 5GC to the NR. This is becausean LTE eNB may support an interface to the 5GC, or may not support aninterface to the 5GC.

The MME or the AMF decides whether the data forwarding is possible.Here, the data forwarding refers to indirect data forwarding. If the MMEdecides that the indirect data forwarding is impossible, the MME informsthe AMF of the information.

Step 1404: The AMF transmits a PDU handover request message to theselected SMF. This message contains a PDN connection and an AMFidentifier. The PDN connection provides the common address of the SMFand the PGW control plane function. According to the information in thereceived relocation required message, the AMF knows that the handover isa handover between different systems, and the AMF requests the SMF toprovide an SM context. The AMF transmits this message to each SMFserving the UE.

Step 1405: The SMF modifies the UPF.

Step 1406: The SMF transmits a PDU session handover response message tothe AMF. This message contains the PDU session identifier, a list ofsuccessfully established EPSs, and Qos rules.

This message further contains the mapping between an EPS bearer and aQos flow in the PDU session. When the AMF requests for the SM context,the SMF also always feeds back the mapped EPS bearer context (if any) tothe AMF. Or, the SMF transmits the mapped EPS bearer context to the AMFonly when the AMF also requests for the mapped EPS context.

Step 1407: The AMF transmits a handover request message to the NG-RAN.This message contains PDU session information to be established. Theinformation about the PDU session contains a session identifier, sessionQos information, Qos flow information, uplink tunnel information foreach session, a source-to-target transparent transmitter and/or Qosinformation of a Qos flow. This message contains the handover type. Thehandover type contains a handover in the LTE, a handover from the LTE tothe NR or more. The handover from the LTE to the NR can further containa handover from an LTE base station connected to the EPC to the NR or ahandover from an LTE base station connected to the 5GC to the NR. Thisis because an LTE eNB may support an interface to the 5GC, or may notsupport an interface to the 5GC.

This message can further contain the mapping between a Qos flow in thePDU session and an EPS bearer, i.e., the identifier of the EPS bearermapped by a Qos flow and/or the mapped Qos information.

Step 1408: The NG-RAN transmits a handover request acknowledgementmessage to the AMF. This message contains one or more pieces of thefollowing information:

a target-to-source transparent transmitter, wherein the target-to-sourcetransparent transmitter can further contain the mapping between a Qosflow in the PDU session and an EPS bearer, i.e., the identifier of theEPS bearer mapped by a Qos flow and/or the mapped Qos information; thetarget-to-source transparent transmitter can further contain informationabout a successfully established PDU session; and the PDU sessioninformation comprises a PDU session identifier and a list of identifiersof successfully established Qos flow in the PDE session;

a list of PDU session information accepted to be established by theNG-RAN, wherein the list of PDU session information contains the PDUsession identifier, downlink tunnel information for the PDU session overan NG3 interface, Qos flow information accepted in the PDU session,rejected Qos flow information, and tunnel information for dataforwarding over the NG3 interface; for a successfully established Qosflow in a successfully established PDU session, if the a 5G-RAN hasreceived a suggestion of downlink data forwarding given by the sourcebase station, the 5G-RAN allocates, to the corresponding PDU session,tunnel information for data forwarding over the NG3 interface; and, fora successfully established Qos flow, if the 5G-RAN has received aproposal of downlink data forwarding given by the source base stationand the data forwarding is possible, the 5G-RAN allocates, to thecorresponding PDU session, tunnel information for data forwarding overthe NG3 interface;

a list of PDU session information rejected to be established by theNG-RAN, wherein the list of PDU session information contains the PDUsession identifier and the causes for rejection.

Step 1409: The AMF transmits a PDU session modification request messageto the SMF. If the tunnel information for data forwarding over the NG3interface has been received from the NG-RAN, the AMF requests the SMF tocreate a data forwarding tunnel. The AMF transmits, to the SMF, thetunnel information for data forwarding received from the NG-RAN. Thismessage contains the PDU session to which the EPS bearer belongs. Thismessage can further contain the mapping between an EPS bearer and a Qosflow in the PDU session.

Step 1410: The SMF transmits a PDU session modification response messageto the AMF. This message contains the tunnel information for dataforwarding between the SGW and the anchor UPF allocated by the SMF orthe anchor UPF. This message contains a target-to-source transparenttransmitter.

This message contains Qos flow information to be mapped in the PDUsession in the 5GS. The Qos flow information contains a Qos flowidentifier and/or Qos information corresponding to the Qos flow.

Step 1411: The SMF transmits, through an N4 session establishment or N4session modification process and to the anchor UPF, the tunnelinformation for downlink data forwarding over the NG 3 interfaceallocated by the NG-RAN. The SMF allocates tunnel information for dataforwarding between the SGW and the UPF. Or, the anchor UPF allocatestunnel information used for data forwarding between the SGW and theanchor UPF and then transmits the tunnel information to the SMF. The N4session establishment message or N4 session modification messagecontains the PDU session to which the EPS bearer belongs. The N4 sessionestablishment message or N4 session modification message can furthercontain the mapping between an EPS bearer and a Qos flow in the PDUsession.

The SMF transmits, to the AMF, the tunnel information used for dataforwarding between the SGW and the anchor UPF.

The PDU session modification response message in step 1410 can beexecuted before or after the N4 session establishment response messageor N4 session modification response message in step 1411.

There are three methods for performing data forwarding between the SGWand the anchor UPF.

Method 1: The method for data transmission between the SGW and theanchor UPF is that each EPS bearer in each PDU session corresponds toone tunnel. The anchor UPF transmits, through a same tunnel and to theNG-RAN, the data of a same PDU session received from the SGW by a tunnelcorresponding to each EPS bearer, that is, the anchor UPF performs amapping from multiple tunnels to one tunnel. In this data forwardingmethod, the anchor UPF allocates, to each EPS bearer requiring dataforwarding in each PDU session, tunnel information used for dataforwarding between the SGW and the anchor UPF. For each PDU session, thenumber of data forwarding tunnels is equal to the number of EPS bearers.The anchor UPF knows, according to the information received from the AMFin step 1409, the number of EPS bearers requiring data forwarding ineach PDU session on the EPS side. The anchor UPF transmits, to the AMF,the tunnel information for data forwarding allocated to each EPS bearerin the PDU session.

Method 2: The method for data transmission between the SGW and theanchor UPF is that each PDU session corresponds to one tunnel. The SGWtransmits, through a same tunnel and to the anchor UPF, the data of asame PDU session received by a tunnel corresponding to each EPS bearer.In this data forwarding method, the anchor UPF allocates, to each PDUsession, tunnel information used for data forwarding between the SGW andthe anchor UPF.

Method 3: The method for data transmission between the SGW and theanchor UPF is that each PDU session corresponds to one tunnel. The SGWtransmits, through a same tunnel and to the anchor UPF, the data of asame PDU session received by a tunnel corresponding to each E-RAB, andthe Qos and/or flow related information is added on a header of the datapacket. In this data forwarding method, the anchor UPF allocates, toeach PDU session, tunnel information used for data forwarding betweenthe SGW and the anchor UPF. In the response message in steps 1411 and1410, the anchor UPF transmits, through the SMF and to the AMF, the Qosflow information to be mapped in the PDU session in the 5GS. The Qosflow information contains a Qos flow identifier and/or Qos informationcorresponding to the Qos flow. The AMF informs the MME of theinformation by step 1412, and the MME informs the SGW of the informationby step 1413. The SGW can contain the Qos and/or flow relatedinformation in the header of the data packet.

The anchor UPF transmits the allocated data forwarding tunnelinformation to the AMF through the SMF.

Step 1412: The AMF transmits a forwarding relocation response message tothe MME. This message contains a target-to-source transparenttransmitter and an EPS bearer setup list.

Step 1413: The MME transmits an indirect data forwarding tunnelestablishment request message to the SGW. This message contains thetunnel information used for data forwarding between the SGW and theanchor UPF.

In the third data forwarding method, this message contains the Qos flowinformation to be mapped by the EPS bearer in the PDU session in the 5GSand is transmitted to the SGW. The Qos flow information contains a Qosflow identifier and/or Qos information corresponding to the Qos flow.

The SGW transmits a indirect data forwarding tunnel establishmentresponse message to the MME. This message contains the uplink tunnelinformation for data forwarding over the S1 interface allocated by theSGW.

Step 1414: The MME transmits a handover command message to the E-UTRAN.This message contains a target-to-source transparent transmitter and thetunnel information for data forwarding over the S1 interface. Thismessage contains the handover type. The handover type contains ahandover in the LTE, a handover from the LTE to the NR or more. Thehandover from the LTE to the NR can further contain a handover from anLTE base station connected to the EPC to the NR or a handover from anLTE base station connected to the 5GC to the NR. This is because an LTEeNB may support an interface to the 5GC, or may not support an interfaceto the 5GC.

Step 1415: The E-UTRAN transmits an E-UTRAN handover command message tothe UE.

This message can further contain the mapping between a Qos flow in thePDU session and an EPS bearer, i.e., the identifier of the EPS bearermapped by a Qos flow and/or the mapped Qos information.

The E-UTRAN forwards data to the SGW. The E-UTRAN forwards data to theSGW on a tunnel corresponding to each E-RAB requiring data forwarding.

The SGW forwards data to the anchor UPF. Corresponding to the three dataforwarding methods described in step 1411, the SGW has differentbehaviors.

In the method 1, the SGW forwards data to the anchor UPF on a tunnelcorresponding to each EPS bearer requiring data forwarding.

In the method 2, the SGW transmits, through a same tunnel and to theanchor UPF, the data of a same PDU session received by the tunnelcorresponding to each EPS bearer. The anchor UPF performs a mapping fromthe PDU session to the Qos flow.

In the method 3, the SGW transmits, through a same tunnel and to theUPF, the data of a same PDU session received by the tunnel correspondingto each E-RAB, and the Qos and/or flow related information is added on aheader of the data packet. The SGW performs a mapping from the PDUsession to the Qos flow. The SGW performs a mapping from the PDU sessionto the Qos flow according to the information received in step 1413.

The anchor UPF forwards data to the NG-RAN. Corresponding to the threedata forwarding methods described in step 1411, the anchor UPF hasdifferent behaviors.

In the method 1, the anchor UPF transmits, through a same tunnel and tothe NG-RAN, the data of a same PDU session received from the SGW by thetunnel corresponding to each EPS bearer to the NG-RAN through a tunnelcorresponding to the PDU session, that is, the anchor UPF performs amapping from multiple tunnels to one tunnel. The anchor UPF forwardsdata to the NG-RAN according to the session transmission method in the5GS, for example, how many flows being used to transmit downlink data ineach PDU session, and how a header of a flow being set. For anunsuccessfully established Qos flow received in step 1408, if the anchorUPF has received the data forwarded by the SGW, the anchor UPF discardsthe data. This is because the access control of the NG-RAN is performedaccording to the Qos flow; however, on the E-UTRAN side, the dataforwarding tunnel corresponds to each E-RAB, and the data forwarding isalso performed in E-RAB level. The level of Qos flow is finer than thelevel of E-RABs. Since the E-URAN cannot distinguish data from differentQos flows or the E-UTRAN does not known the information about anunsuccessfully established Qos flow, the E-UTRAN may forward the data ofan unsuccessfully established Qos flow in the E-RAB to the SGW, and theSGW then transmits the data to the anchor UPF.

In the method 2, the anchor UPF directly receives data from the SGW bythe tunnel corresponding to each PDU session. The anchor UPF forwardsdata to the NG-RAN according to the session transmission method in the5GS, for example, how many flows being used to transmit downlink data ineach PDU session, and how a header of a flow being set.

In the method 3, the anchor UPF directly receives, from the SGW, thedata to be forwarded in the 5GS. The anchor UPF forwards data to theNG-RAN.

During the PDU session establishment or EPS bearer establishmentprocess, the UE receives, from the network, Qos information and/or Qosflow information of the Qos flow mapped by the EPS bearer. The UEcorrelates the ongoing EPS bearers with the Qos flows in the PDU sessioncontained in the handover command message. For an EPS bearer without thecorresponding Qos flow, the UE can delete this EPS bearer.

Or, the UE obtains, from the handover command message, the mappingbetween a Qos flow in the PDU session and an EPS bearer. The UEcorrelates the ongoing EPS bearers with the Qos flows contained in thehandover command message. For an EPS bearer without the correspondingQos flow, the UE can delete this EPS bearer.

Step 1416: The UE transmits a handover completion message to the NG-RAN.

Step 1417: The NG-RAN transmits a handover notification message to theAMF. This message contains the tunnel information for downlink datatransmission allocated by the NG-RAN.

Step 1418: The AMF transmits a handover completion message to the SMF.

Step 1419: The SMF transmits an N4 session modification message to theUPF. The UPF transmits the N4 session modification response to the SMF.The AMF transmits the tunnel information for downlink data transmissionallocated by the NG-RAN to the anchor UPF through the SMF.

Step 1420: The SMF transmits a session handover completionacknowledgement message to the AMF.

The session modification response message in step 1420 can be executedbefore or after the N4 session modification response message in step1419.

So far, the process of supporting the data forwarding during thehandover between different systems can be well supported by using theinformation obtained from intra-5GS handover procedure in the presentinvention. By this method, the handover problem from the EPS to the 5GScan be solved, so that the loss of data is avoided and the continuity ofservices is ensured.

The present invention provides a method for supporting handover.Specifically, when a PDU session of a dual-registered UE is transferredfrom an EPS to a 5GS, the method comprises the following steps of:

based on a PDU session request initiated by a UE, establishing a PDUsession; and

based on the PDU session, acquiring tunnel information for dataforwarding, and then performing data forwarding.

As shown in FIG. 15 , this method is a method for supporting dataforwarding when a PDU session of a UE is to be handed over from the EPSto the 5GS in the case of dual registration. This method comprises thefollowing steps.

Step 1501: A UE transmits a Non-Access Stratum (NAS) message to an AMFto initiate a PDU session establishment process. The NAS messagecontains the PDU session identifier and N1 Session Management (SM)information. The N1 SM information contains a PDU session establishmentrequest message. This message contains a handover indication.

An NG-RAN transmits the received NAS message to the AMF through an NGmessage. The NG message further contains the location information of auser and the type of the access technology.

Step 1502: For a new PDU session to be established, the AMF selects anSMF. The AMF saves the PDU session identifier and the identifier of theSMF. If the received message contains the handover indicationinformation, the AMF use an SMF obtained from the subscription data. TheAMF saves the received handover indication.

Step 1503: The AMF transmits an SM request message to the SMF. Thismessage contains the N1 SM information received from the UE. Thismessage contains a handover indication. The SMF saves the receivedhandover indication information.

Step 1504: The SMF triggers a PDU session authentication andauthorization process.

Step 1505: If the PDU session establishment request is an initialrequest and is not executed in step 1504, the SMF transmits an N4session establishment message to the selected UPF. Otherwise, the SMFtransmits an N4 session modification request message to the selectedUPF.

Step 1506: The UPF transmits an N4 session establishment responsemessage or an N4 modification response message to the SMF.

Step 1507: The SMF transmits an SM response message to the AMF. Thismessage contains N2 SM information and N1 SM information. The N1 SMinformation contains an NAS message PDU session establishment acception.The N2 SM information contains the PDU session identifier, Qos profile,core network tunnel information and S-NSSAI. The N2 SM informationcontains the EPS Qos information mapped by the Qos flow in the PDUsession. The N1 SM information contains the EPS Qos information mappedby the Qos flow in the PDU session.

If there is the handover indication, the SMF determines, according tothe Qos of a service, whether the PDN connection requires dataforwarding. The SMF can determine the data forwarding for each PDUsession or whether each Qos flow in the PDU session requires dataforwarding. If the data forwarding is required, the SMF contains a dataforwarding indication in the SM response message. The data forwardingindication can be specific to the data forwarding of the PDU session orspecific to each Qos flow in the PDU session.

Step 1508: The AMF transmits a PDU session resource establishmentrequest message to an NG-RAN. The message contains the N2 SM informationand the NAS message PDU session establishment acception. The NG-RANsaves the PDU session information, information of the Qos flow in theDPU session, and/or the EPS Qos information mapped by the Qos flow. TheAMF contains the data forwarding indication in this message. The dataforwarding indication can be specific to the data forwarding of the PDUsession or specific to each Qos flow in the PDU session. The PDU sessionresource establishment request message can also be an initial contextestablishment request message.

Step 1509: The NG-RAN transmits an access network resource establishmentrequest message to the UE. The access network resource establishmentrequest message can be an RRC connection reconfiguration message. TheNG-RAN allocates NG interface user plane downlink tunnel information.The message contains the NAS message PDU session establishment acceptionmessage. If the NG-RAN can allocate the required resources and thetunnel information of the NG-RAN side, the NG-RAN transmits an NASmessage to the UE. The RRC message or the NAS message contains the EPSbearer identifier and/or EPS Qos information mapped by a Qos flow.

Step 1510: The UE transmits an access network resource establishmentmessage to the NG-RAN. The access network resource establishment messagecan be an RRC connection reconfiguration completion message. Thismessage can further contain an NAS message PDU session establishmentcompletion message.

Step 1511: The NG-RAN transmits a PDU session resource establishmentresponse message to the AMF. This message contains the N2 SM informationand the PDU session identifier. The N2 SM information contains the PDUsession identifier, RAN tunnel information, the accepted Qos informationand/or the rejected Qos flow information. This message or the N2 SMinformation further contains the EPS bearer identifier mapped by a Qosflow. This message or the N2 SM information further contains the NASmessage PDU session establishment completion message.

If the NG-RAN has received the data forwarding indication information,the NG-RAN allocates, to the PDU session or each Qos flow in the PDUsession, tunnel information for downlink data forwarding. The tunnelinformation contains a tunnel identifier and the address of a transportlayer. The PDU session resource establishment response message containsthe data forwarding tunnel information allocated by the NG-RAN.

Step 1512: The AMF transmits an SM request message to the SMF. Thismessage contains the N2 SM information received from the NG-RAN. Thismessage contains the NAS message received from the UE. This messagecontains the EPS bearer identifier mapped by a Qos flow. This messagecontains the data forwarding tunnel information received from theNG-RAN.

Step 1513: If the N4 session of the corresponding PDU session has notbeen established, the SMF initiates an N4 session establishment processto the UPF. Otherwise, the SMF initiates an N4 session modificationprocess. The SMF transmits the tunnel information allocated by theaccess network and/or the core network tunnel information to the UPF.The SMF further transmits, to the UPF, the data forwarding tunnelinformation received from the NG-RAN.

If the UPF has received the data forwarding tunnel information, the SMFor the UPF allocates the tunnel information for data forwarding betweenthe SGW and the UPF, corresponding to the PDN session transferred fromthe EPS to the 5GS. If the tunnel information is allocated by the UPF,the UPF transmits the allocated tunnel information to the SMF throughthe N4 session modification message. The tunnel information can bespecific to each EPS bearer or each PDU session. In order to support themovement between different systems, the SMF further has a function of aPGW control plane. The SMF transmits, to the SGW, the tunnel informationfor direct data forwarding between the SGW and the UPF. The SGWallocates the tunnel information for data forwarding between the eNB andthe SGW, and then transmits the tunnel information to the eNB throughthe MME. The tunnel information is specific to each EPS bearer.

If the eNB has received the data forwarding uplink tunnel information,the eNB forwards downlink data to the SGW. According to whether thereceived data cannel information is specific to each EPS bearer or eachPDU session, the SGW forwards data to the UPF. The UPF transmits,according to the relationship between the EPS bearer and the PDU sessionand/or the mapping between an EPS bearer and a Qos flow in the PDUsession, the data of the EPS bearer to the NG-RAN by a tunnel of themapped PDU session.

The NG-RAN transmits the forwarded data to the UE, and then transmitsother data.

Step 1514: The UPF transmits an N4 session modification response messageor an N4 session request response message to the SMF.

Step 1515: The SMF transmits an SM response message to the AMF.

So far, the thirteenth method for supporting handover according to thepresent invention has been described. By this method, during thetransfer of a PDU session of a dual-connection UE from the 5GC to theEPC, the data forwarding can be well supported, so that the loss of datais avoided, the delay of data forwarding is reduced and the continuityof services is ensured.

FIG. 16 shows an EPS bearer identifier allocation method for supportinghandover between different systems according to the present invention.This method can well support the data forwarding during the handoverbetween different systems. This method comprises the following steps.

Step 1601: An NG-RAN decides to hand over a UE to an E-UTRAN.

Here, the E-UTRAN can be an eNB connected to the EPC. The NG-RAN can bea gNB, an eNB connected to the 5GC or a Centralized Unit (CU) in thegNB.

The user plane path before handover is from an UPF to the NG-RAN. TheSGW needs to support an interface to the UPF. The UPF can contain afunction of a PGW user plane and a function of performing user planeanchoring during the handover between different RATs.

Each PDU session contains one or more Qos flows. The UE has one or moreongoing PDU sessions. During a PDU session establishment or GuaranteedBusiness Rate (GBR) Qos flow establishment process, mapped EPS Qosinformation is allocated to a Qos flow. A Non-GBR Qos flow is mapped toa default EPS bearer. A GBR Qos flow is mapped to a dedicated EPSbearer. The EPS Qos information mapped by a Qos flow can be allocated bya PCC or an SMF. In order to support the handover between differentsystems, an SMF can have functions of a PGW control plane. In a PCCdeployment scenario, a Policy Control Function (PCF) provides the SMFwith mapped EPS Qos of a Qos flow. In order to support the handoverbetween different systems, the PCF can have a Policy Control andCharging Rules Function (PCRF). The SMF transmits, through the AMF andto the UE, the EPS Qos information mapped by the Qos flow, for example,through a non-access stratum message PDU session establishment message.During the PDU session establishment or GBR Qos flow establishmentprocess, the SMF can further transmit, through the AMF and to theNG-RAN, the EPS Qos information mapped by the Qos flow. The AMFtransmits the EPS Qos information mapped by the Qos flow to the NG-RANthrough an initial context establishment request message or a PDUsession resource establishment request message. The NG-RAN can transmitthe EPS Qos information mapped by the Qos flow to the UE through an RRCmessage. The E-RAB and the EPS bearer are identical. It is called theEPS bearer in a core network, while it is called the E-RAB in an accessnetwork.

The NG-RAN allocates the EPS bearer identifier mapped by a Qos flow inthe PDU session. The NG-RAN can ensure that the allocated EPS beareridentifiers are unique and the number of the allocated EPS beareridentifiers does not exceed the maximum limit (eight). The E-RABidentifier and the EPS bearer identifier are identical. It is called theEPS bearer identifier in a core network, while it is called the E-RABidentifier in an access network.

Step 1602: The NG-RAN transmits a handover required message to an AMF.This message contains the identifier of a target eNB and asource-to-target transparent transmitter. The source-to-targettransparent transmitter contains the E-RAB identifier and a downlinkdata forwarding proposal for the E-RAB. The NG-RAN obtains the EPS Qosinformation mapped by the Qos flow in the PDU session through a PDUsession establishment process or a handover process. The method forobtaining, by the NG-RAN, the information through a handover process isas described in the method shown in FIG. 3 . The method for obtaining,by the NG-RAN, the EPS bearer information through a PDU sessionestablishment process is as described in step 1601. The NG-RAN decides,according to the mapping from the Qos flow to the E-RAB and the Qosinformation, whether to propose data forwarding. The NG-RAN can make adecision by considering other factors, for example, the presence orabsence of data in a buffer, without influencing the main contents ofthe present invention.

The handover required message can further contain a list of EPS bearerinformation. The EPS bearer information contains the EPS beareridentifier and/or the Qos information of the EPS bearer. The handoverrequired message can further contain the mapping between a Qos flow inthe PDU session and an EPS bearer identifier. In the present invention,the mapping between a Qos flow and an EPS bearer identifier means thatthere is no EPS bearer identifier corresponding to the Qos flow in thePDU session.

This message further contains identification information indicating anMME to which the target eNB is connected. The identification informationcan be an identifier of a tracked area or an identifier of the MME.

The NG-RAN informs the AMF of the handover type. The handover typeincludes a handover in NR, a handover from NR to LTE, a handover from NRto UTRAN and a handover from NR to GERAN and/or GSM. For the handoverfrom NR to LTE, the NG-RAN informs the AMF whether the target basestation for the handover is a base station connected to a 5G corenetwork or whether this handover is a handover between differentsystems. This is because an LTE eNB may support an interface to the 5GC,or may not support an interface to the 5GC. If the target base stationis also connected to the 5GC, the handover is a handover in the 5Gsystem. If the target base station is connected to the EPC but notconnected to the 5GC, the handover is a handover between differentsystems. The NG-RAN can contain, in the handover required message,information about an inter-system handover or information indicatingthat there is no interface between the target base station and the 5GC,so as to inform the AMF that this handover is a handover betweendifferent systems. If an identifier of an eNB connected to the 5GC andan identifier of an eNB not connected to the 5GC are different inlength, the 5GC can determine, according to the length of the identifierof the target base station contained in the received handover requiredmessage, whether the handover is a handover between different systems.If the identification information of the MME connected to the targetbase station and the identifier of the AMF node are defined differently(for example, different in length), the 5GC can determine, according tothe length of the identifier of the core network connected to the targetbase station contained in the received handover required message,whether the handover is a handover between different systems. Or, theNG-RAN directly sets the handover type in the handover required messageas a handover from the NR to an eNB connected to the EPC or a handoverfrom the NR to an eNB connected to the 5GC, to inform the AMF of thehandover type. The handover type indicates the core network to which thetarget eNB is connected is an EPC or a 5GC. If the core network is a5GC, the handover is an intra-system handover. If the core network is anEPC, the handover is a handover between different systems.

Step 1603 a: The AMF transmits a Session Management (SM) context requestmessage to an SMF. According to the received information in the handoverrequired message, the AMF knows that the handover is a handover betweendifferent systems, and the AMF requests the SMF to provide an SMcontext. The AMF can further request for an EPS bearer context. The AMFtransmits this message to each SMF serving the UE. The AMF transmits, tothe SMF, the EPS bearer identifier and/or the Qos information of the EPSbearer received from the NG-RAN. This message can further contain themapping between a Qos flow in the PDU session and an EPS beareridentifier.

Step 1603 b: The SMF transmits an SM context response message to theAMF. This message contains the SM context of the UE. The SM contextfurther contains the mapped EPS bearer context, for example, the EPSbearer identifier and/or EPS Qos information. When the AMF requests forthe SM context, the SMF also always feeds back the mapped EPS bearercontext (if any) to the AMF if there is a mapped EPS bearer context. Or,if the AMF also requests for the mapped EPS bearer context when itrequests for the SM context to the SMF, the SMF transmits the mapped EPSbearer context to the SMF. According to the information, which isreceived from the source NG-RAN, indicating that there is no connectionbetween the target eNB and the 5G core network or indicating that thehandover is a handover between different systems or a handover from theNR to an eNB connected to the EPC, the AMF knows that the handover is ahandover between different systems, and the AMF requests the SMF toprovide the mapped EPS bearer context information. This message canfurther contain the mapping between a Qos flow in the PDU session and anEPS bearer identifier.

In this method of the present invention, steps 1603 a and 1603 b may notbe executed. The AMF acquires, from the handover required messagereceived from the NG-RAN, EPS bearer information in the PDU session, forexample, the EPS bearer identifier and EPS Qos information, so that theAMF can constitute a relocation request message in step 1604.

Step 1604: The AMF transmits a relocation request message to an MME.According to the identification information indicating the MME connectedto the target eNB contained in the handover request message, the AMFselects and finds an MME. The identification information of the MMEconnected to the target eNB can be a TAI. This message contains theidentifier of the target eNB, a source-to-target transparent transmitterand mapped EPS UE context information. The mapped EPS UE contextinformation contains UE Mobility Management (MM) context information andSession Management (SM) context information.

The AMF or the MME decides whether the data forwarding is possible.Here, the data forwarding refers to indirect data forwarding. If the AMFdecides that the indirect data forwarding is impossible, the AMF informsthe MME of the information.

The AMF informs the MME of the information of the Qos flow(s) containedin the PDU session. The relocation request message can further containthe mapping between a Qos flow in the PDU session and an EPS beareridentifier.

Step 1605: The MME transmits a session establishment request message toan SGW. This message contains the EPS bearer context information.

Step 1606: The SGW transmits a session establishment response message tothe MME. This message contains tunnel information for uplink datatransmission over an S1 interface allocated by the SGW.

Step 1607: The MME transmits a handover request message to the E-UTRAN.This message contains a source-to-target transparent transmitter and anE-EAB context. The E-RAB context contains an E-RAB to be established anduplink tunnel information over the S1 interface allocated by the SGW.The E-RAB context contains information indicating whether the dataforwarding is possible. This message contains the handover type. Thespecific content is the same as that in step 1602 and will not berepeated here. The handover request message can further contain themapping between a Qos flow in the PDU session and an EPS beareridentifier.

Step 1608: The E-UTRAN transmits a handover request acknowledgementmessage to the MME. This message contains a list of established E-RABs,a list of unsuccessfully established E-RABs and a target-to-sourcetransparent transmitter. For the established RABs, this message furthercontains tunnel information for downlink data transmission over the S1interface. For the established E-RABs, if the source base station givesa proposal of downlink data forwarding, the data forwarding is possibleand the target eNB accepts the downlink data forwarding proposal, thetarget base station contains the tunnel information which is allocated,by the E-UTRAN to each E-RAB requiring downlink data forwarding, fordata forwarding over the S1 interface. The target-to-source transparenttransmitter can further contain the mapping between a Qos flow in thePDU session and an EPS bearer identifier.

Step 1609: The MIME requests the SGW to establish an indirect dataforwarding tunnel. This step is executed only when the indirect dataforwarding is to be executed. Upon receiving, from the E-UTRAN, thedownlink tunnel information for data forwarding over the S1 interface,the MME requests the SGW to establish an indirect data forwardingtunnel. The MME transmits, to the SGW, the address of a transport layerfor data forwarding and a TEID allocated by the eNB. The address of thetransport layer and the TEID correspond to each E-RAB.

The SGW transmits an indirect data forwarding tunnel establishmentresponse message to the MIME. This message contains the information fordata forwarding between the SGW and the UPF allocated by the SGW. Theinformation for data forwarding between the SGW and the UPF contains aPDU session identifier and/or E-RAB information contained in the PDUsession. The E-RAB information contains an E-RAB identifier and thetunnel information used by the E-RAB for data forwarding. The tunnelinformation contains the address of a transport layer and a TEIDallocated by the SGW. The SGW allocates downlink data forwarding tunnelinformation to an E-RAB requiring downlink data forwarding. The SGWallocates uplink data forwarding tunnel information to an E-RABrequiring uplink data forwarding. The data forwarding tunnel informationcontained in the E-RAB information can contain uplink and/or downlinkdata forwarding tunnel information.

There are two methods for performing data forwarding between the UPF andthe SGW.

A method for data transmission between the UPF and the SGW is that eachEPS in each PDU session bears one tunnel. If the NG-RAN and the UPFperform data forwarding in such a way that each PDU session correspondsto one user plane tunnel, the UPF transmits, to the SGW and through atunnel corresponding to an EPS bearer of each Qos flow, data of multipleQos flows belonging to a same PDU session received from the NG-RAN, thatis, the UPF performs a mapping from one tunnel to multiple tunnels. TheUPF performs, according to the correspondence between a Qos flow in thePDU session and an EPS bearer identifier or an E-RAB, a mapping from onetunnel to multiple tunnels. In this forwarding method, the SGWallocates, to each EPS bearer requiring data forwarding in each PDUsession, a tunnel information used for data forwarding between the SGWand the UPF. For each PDU session, the number of data forwarding tunnelsis equal to the number of EPS bearers. The UPF knows, according to theinformation received from the SMF in step 412, the number of EPS bearersrequiring data forwarding in each PDU session in the EPS, the mappingrelationship between a Qos flow and an EPS bearer and/or the EPS beareridentifier. The UPF knows information of the Qos flow(s) of the PDUsession contained in the 5G system.

The step 1609 is executed only when the indirect data forwarding ispossible.

Step 1610: The MME transmits a relocation response message to the AMF.This message contains the tunnel information allocated by the SGW fordata forwarding. The tunnel information is specific to each EPS bearerin each PDU session. This message contains a target-to-sourcetransparent transmitter. The MME transmits, to the AMF, the E-RABinformation contained in the PDU session and the tunnel information fordata forwarding allocated by the SGW to each E-RAB.

This message contains the information for data forwarding between theSGW and the UPF allocated by the SGW. The information for dataforwarding between the SGW and the UPF contains a PDU session identifierand/or E-RAB information contained in the PDU session. The E-RABinformation contains an E-RAB identifier and the tunnel information usedby the E-RAB for data forwarding. The information for data forwardingbetween the SGW and the UPF contains the tunnel information for dataforwarding allocated to each E-RAB of each PDU session by the SGW. Thetunnel information for data forwarding can contain uplink and/ordownlink data forwarding tunnel information.

The MME directly transmits the E-RAB information to the AMF, and theE-RAB information is transferred by the AMF.

Step 1611: The AMF requests the SMF to create a data forwarding tunnel.The AMF transmits a indirect data forwarding tunnel establishmentrequest message to the SMF. This message contains PDU sessioninformation. The PDU session information contains the PDU sessionidentifier, information of the Qos flow contained in the PDU session,the number of EPS bearers requiring data forwarding in each PDU sessionin the EPS, the mapping between a Qos flow and an EPS bearer, the EPSbearer identifier and/or Qos information of each EPS bearer. Thismessage contains the information for data forwarding received from theMME.

Step 1612: The SMF transmits an N4 session modification message to theUPF. This message contains PDU session information. The PDU sessioninformation contains the PDU session identifier, information of the Qosflow(s) contained in the PDU session, the number of EPS bearersrequiring data forwarding in each PDU session in the EPS, the mappingbetween a Qos flow and an EPS bearer, the EPS bearer identifier and/orQos information of each EPS bearer. This message contains theinformation for data forwarding received from the AMF.

The N4 session modification message contains EPS bearer informationcontained in the PDU session. The EPS bearer information contains an EPSbearer identifier and tunnel information used by each EPS bearer fordata forwarding. The SMF informs the UPF of the correspondence between aQos flow and an EPS bearer in the PDU session. The UPF knows the Qosflow information of the PDU session in the 5G system, and the UPFreceives, from the SMF, the EPS bearer information contained in the PDUsession and the mapping between a Qos flow and an EPS bearer.

The UPF allocates tunnel information for data forwarding between theNG-RAN and the UPF and then transmits the tunnel information to the SMF.

A method for data forwarding between the NG-RAN and the UPF is asfollows.

The NG-RAN and the UPF perform data forwarding in such a way that eachPDU session corresponds to one user plane tunnel. In this dataforwarding method, the UPF allocates tunnel information for each PDUsession. The tunnel information contains the address of a transportlayer and a TEID.

The UPF transmits the allocated tunnel information for data forwardingto the SMF. The SMF receives an N4 session modification response messagefrom the UPF. This message contains the tunnel information for dataforwarding between the NG-RAN and the UPF allocated by the UPF.

Step 1613: The SMF transmits a indirect data forwarding tunnelestablishment response message to the AMF. This message contains thetunnel information for data forwarding between the NG-RAN and the UPFallocated by the UPF.

Step 1614: The AMF transmits a handover command message to the NG-RAN.This message contains a target-to-source transparent transmitter and thetunnel information for data forwarding allocated by the UPF. Thismessage further contains information about established PDU sessions andinformation about unsuccessfully established PDU sessions. Theinformation about established PDU sessions contains information aboutestablished Qos flows and information about unsuccessfully establishedQos flows. The tunnel information for data forwarding is specific toeach PDU session.

Step 1615: The NG-RAN transmits a handover command message to the UE.This message can further contain the mapping between the Qos flow in thePDU session and the EPS bearer identifier. The UE correlates the ongoingQos flows with the EPS bearer identifier in the PDU session contained inthe handover command message. For a Qos flow without the correspondingEPS bearer, the UE can delete this Qos flow.

The NG-RAN forwards data to the UPF. For a PDU session requiring dataforwarding and on a corresponding tunnel, the NG-RAN forwards data tothe UPF.

The NG-RAN transmits, on a user plane tunnel allocated for the PDUsession, data of each Qos flow to the UPF. For the downlink data, theNG-RAN transmits, on a tunnel allocated for downlink data forwarding, adownlink data packet to the UPF.

The UPF forwards data to the SGW. The UPF directly forwards the datareceived from the NG-RAN to the SGW through a user plane tunnelallocated for the corresponding EPS bearer. The SGW directly forwardsthe data to the target base station. The UPF forwards, according to themapping between a Qos flow and an EPS bearer and through the user planetunnel allocated for the corresponding EPS bearer, data of different Qosflows in the PDU session to the SGW. According to the mapping between aQos flow and an EPS bearer and the information about the EPS beareraccepting the data forwarding, the UPF knows a Qos flow accepting thedata forwarding, and the UPF forwards, to the SGW and through the userplane tunnel allocated for the corresponding EPS bearer, the data of theQos flow that accepts the data forwarding. Correspondingly, if there isno Qos flow accepting the data forwarding and there is no correspondingdata forwarding tunnel, the UPF discards the data. The SGW directlyforwards data to the target base station.

The SGW forwards data to the E-UTRAN. The SGW transmits, to the E-UTRANand through the corresponding tunnel allocated by the E-UTRAN, the datareceived from the UPF by the tunnel corresponding to each EPS bearer,that is, the UPF performs a mapping from multiple tunnels to one tunnel.The SGW forwards data to the E-UTRAN according to the sessiontransmission method in the EPS.

Step 1616: The UE transmits a handover completion message to theE-UTRAN.

Step 1617: The E-UTRAN transmits a handover completion message to theMME. This message contains the tunnel information allocated by theE-UTRAN for downlink data transmission.

Step 1618: The MME transmits a bearer modification request message tothe SGW. This message contains the tunnel information for downlink datatransmission over the S1 interface.

Step 1619: The SGW transmits a bearer modification request message tothe SMF. The SMF can further have a function of a PGW control plane. TheSGW allocates tunnel information for downlink data transmission betweenthe SGW and the UPF, and the tunnel information corresponds to each EPSbearer or each PDU session.

Step 1620: The SMF requests the UPF to perform session modification. TheSMF can further have a function of a PGW control plane. The SMFtransmits, to the UPF, the tunnel information for downlink datatransmission between the SGW and the UPF allocated by the SGW, and thetunnel information corresponds to each EPS bearer or each PDU session.The UPF transmits a session modification response to the SMF. The UPFallocates tunnel information for uplink data transmission between theSGW and the UPF, and the UPF transmits, to the SMF, the tunnelinformation for uplink data transmission.

Step 1621: The SMF transmits a bearer modification response message tothe SGW. This message contains the tunnel information for uplink datatransmission between the SGW and the UPF allocated by the UPF.

Step 1622: The SGW transmits a bearer modification response message tothe MME.

So far, the method for allocating, by the NG-RAN, the EPS beareridentifier while supporting the handover between different systemsaccording to the present invention has been completed. The dataforwarding during the handover between different systems can be wellsupported. By this method, the handover problem from the 5GS to the EPScan be solved, so that the loss of data is avoided and the continuity ofservices is ensured. By this method, the EPS bearer identifier isallocated as required, so that unnecessary allocation and transmissionare avoided and the operations to the equipment are simplified.

FIG. 17 shows a schematic flowchart of a second method for handing overa UE from an EPS to a 5GS system according to the present invention.This method comprises the following steps.

Step 1701: An E-UTRAN decides to hand over a UE to an NG-RAN.

Here, the E-UTRAN can be an eNB connected to the EPC. The NG-RAN can bea gNB, an eNB connected to the 5GC or a Centralized Unit (CU) in thegNB. The eNB connected to the 5GC can also be referred to as an ng-eNB.

The user plane path before handover is anchors UPF, SGW and E-UTRAN. TheSGW needs to support an interface to the anchor UPF. The anchor UPF canbe located in the 5GC or the EPC or can be a common entity. The anchorUPF can be a UPF serving the UE, and executes a function of a user planeanchor during the handover between different Radio Access Technologies(RATs). The anchor UPF can be a function of a PGW user plane plus an UPFor a function of an UPF plus a PGW user plane, and executes a functionof a user plane anchor during the handover between different RadioAccess Technologies (RATs).

The UE has one or more ongoing PDU sessions (also referred to as PDNconnections). Each PDU session contains one or more EPS bearers. Duringa PDU session establishment or EPS bearer establishment process, Qosinformation and/or Qos flow identifier of a mapped Qos flow and/or theidentifier of a PDU session are allocated to each EPS bearer. DefaultEPS bearer is mapped to a non-Guaranteed Business Rate (non-GBR) Qosflow. The PDU session to which the EPS bearer belongs can be allocatedby the function of the PCC or PGW control plane. The Qos informationand/or Qos flow identifier of a mapped Qos flow of the EPS bearer can beallocated by a function of a PCC or PGW control plane. In order tosupport the handover between different systems, a PGW control plane canfurther have an SMF function. In a PCC deployment scenario, the PolicyControl and Charging Rules Function (PCRF) provides the SMF with theidentifier of the PDU session to which an EPS bearer belongs, and thePCRF provides the SMF with the Qos and/or Qos flow identifier of a Qosflow mapped by an EPS bearer. In order to support the handover betweendifferent systems, a PCRF further has a Policy Control Function (PCF).The PGW transmits, to the SGW, the identifier of the PDU session towhich an EPS bearer belongs, the Qos information of a Qos flow mapped byan EPS bearer and/or the identifier of a Qos flow mapped by an EPSbearer. The SGW saves the received identifier of the PDU session towhich an EPS bearer belongs. The SGW saves the received Qos and/or Qosflow identifier of a Qos flow mapped by an EPS bearer. The SMF transmitsto the UE via the MME, the Qos and/or Qos flow identifier of a Qos flowmapped by an EPS bearer and/or the identifier of the PDU session towhich an EPS bearer belongs e.g. through a non-access stratum messagePDN connection establishment message. The MME acquires, from the SMF,the Qos and/or Qos flow identifier of a Qos flow mapped by an EPS bearerand/or the identifier of the PDU session to which an EPS bearer belongs.The E-RAB identifier and the EPS bearer identifier are identical or inone-to-one correspondence. It is called an EPS bearer in a core network,while it is called an E-RAB in an access network. The E-RAB identifierand the EPS bearer identifier are identical or in one-to-onecorrespondence. It is called the EPS bearer identifier in a corenetwork, while it is called the E-RAB identifier in an access network.

Step 1702: The E-UTRAN transmits a handover required message to an MME.This message contains the identifier of a target NG-RAN node and asource-to-target transparent transmitter. This message further containsidentification information indicating an AMF to which the target NG-RANis connected. The identification information can be an identifier of atracked area, an identifier of a network slice, an identifier of an AMFpool, an identifier of the AMF, or more.

This message contains the handover type. The handover type contains ahandover in the LTE, a handover from the LTE to the NR or more. Thehandover from the LTE to the NR can further contain a handover from anLTE base station connected to the EPC to the NR or a handover from anLTE base station connected to the 5GC to the NR. This is because an LTEeNB may support an interface to the 5GC, or may not support an interfaceto the 5GC.

The E-UTRAN gives a proposal of data forwarding for an E-RAB. Thehandover required message contains a list of E-RAB information of theUE. The E-RAB information contains an E-RAB identifier and/or thedownlink data forwarding proposed for the E-RAB. Here, the list of E-RABinformation of the UE is directly contained in the handover requiredmessage or in the source-to-target transparent transmitter. If the listof E-RAB information of the UE is contained in the handover requiredmessage, the core network can know the downlink data forwardinginformation proposed for the E-RAB. For the handover between differentsystems, that is, if the target base station is an NG-RAN, the E-UTRANcontains the E-RAB identifier of the UE and the downlink data forwardingin the handover required information or in the source-to-targettransparent transmitter. For the handover in the LTE system, the E-UTRANcontains the E-RAB identifier of the UE and the downlink data forwardingin the source-to-target transparent transmitter.

Step 1703: The MME transmits a relocation request message to an AMF.According to the identification information of an AMF to which thetarget NG-RAN node connects contained in the handover required message,the MME selects and finds the AMF. This message contains the identifierof the target NG-RAN node, a source-to-target transparent transmitterand UE context information. The UE context information contains UEMobility Management (MM) context information and Session Management (SM)context. This message contains the handover type. The handover typecontains a handover in the LTE, a handover from the LTE to the NR ormore. The handover from the LTE to the NR can further contain a handoverfrom an LTE base station connected to the EPC to the NR or a handoverfrom an LTE base station connected to the 5GC to the NR. This is becausean LTE eNB may support an interface to the 5GC, or may not support aninterface to the 5GC.

This message contains a list of EPS bearer information of the UE. TheEPS information contains an EPS identifier and/or the downlink dataforwarding proposed for the EPS bearer.

The MME or the AMF decides whether the data forwarding is possible.Here, the data forwarding refers to indirect data forwarding. If the MMEdecides that the indirect data forwarding is impossible, the MME informsthe AMF of the information.

Step 1704: The AMF transmits a PDU handover request message to theselected SMF. This message contains a PDN connection and an AMFidentifier. The PDN connection provides the common address of the SMFand the PGW control plane function. According to the information in thereceived relocation request message, the AMF knows that the handover isa handover between different systems, and the AMF requests the SMF toprovide an SM context. The AMF transmits this message to each SMFserving the UE.

This message contains a list of EPS bearer information of the UE. TheEPS information contains an EPS identifier and/or the downlink dataforwarding proposed for the EPS bearer.

The SMF receives, from the MME, an EPS bearer identifier and thedownlink data forwarding proposed for the EPS bearer. According to thePDU session identifier and QoS flow identifier(s) corresponding to anEPS bearer obtained during the PDU session or EPS bearer establishmentprocess, the SMF knows the PDU session and Qos flow(s) corresponding toan EPS bearer, so that the SMF can know the PDU session and the Qosflow(s) that the source base station proposed for downlink dataforwarding.

Step 1705: The SMF modifies an UPF.

Step 1706: The SMF transmits a PDU session handover response message tothe AMF. This message contains the PDU session identifier, an EPS bearersetup list, and Qos rules.

This message further contains the mapping between EPS bearers and Qosflows in the PDU session. When the AMF requests for the SM context, theSMF also always feeds back the mapped EPS bearer context (if any) to theAMF. Or, the SMF transmits the mapped EPS bearer context to the AMF onlywhen the AMF also requests for the mapped EPS context.

Step 1707: The AMF transmits a handover request message to the NG-RAN.This message contains PDU session information to be established. Theinformation about the PDU session contains an PDU session identifier,downlink data forwarding for the PDU session, session Qos information,Qos flow information, a downlink data forwarding proposal for the Qosflow, uplink tunnel information of each PDU session, a source-to-targettransparent transmitter, and/or Qos information of a Qos flow. Thismessage contains the handover type. The handover type contains ahandover in the LTE, a handover from the LTE to the NR or more. Thehandover from the LTE to the NR can further contain a handover from anLTE base station connected to the EPC to the NR or a handover from anLTE base station connected to the 5GC to the NR. This is because an LTEeNB may support an interface to the 5GC, or may not support an interfaceto the 5GC.

This message contains a list of Qos flow information in the PDU session.The Qos flow information contains a Qos flow identifier, Qos informationof the Qos flow and/or downlink data forwarding information proposed forthe Qos flow.

This message can further contain the mapping between a Qos flow in thePDU session and an EPS bearer, i.e., the identifier of the EPS bearermapped by a Qos flow and/or the mapped Qos information. This messagecontains a source-to-target transparent transmitter.

Step 1708: The NG-RAN transmits a handover request acknowledgementmessage to the AMF. This message contains one or more pieces of thefollowing information:

a target-to-source transparent transmitter, wherein the target-to-sourcetransparent transmitter can further contain the mapping between a Qosflow in the PDU session and an EPS bearer, i.e., the identifier of theEPS bearer mapped by a Qos flow and/or the mapped Qos information; thetarget-to-source transparent transmitter can further contain informationabout a successfully established PDU session; and the PDU sessioninformation comprises a PDU session identifier and a list of identifiersof a successfully established Qos flow in the PDU session;

a list of PDU session information accepted to be established by theNG-RAN, wherein the list of PDU session information contains the PDUsession identifier, downlink tunnel information for the PDU session overan NG3 interface, Qos flow information accepted for the PDU session, Qosflow information that is not accepted, and tunnel information for dataforwarding over the NG3 interface; for a successfully established Qosflow in a successfully established PDU session, if the 5G-RAN hasreceived the proposed downlink data forwarding and has accepted the dataforwarding, the NG-RAN allocates, to the corresponding PDU session,tunnel information for data forwarding over the NG3 interface; for asuccessfully established Qos flow, if the 5G-RAN has received theproposed downlink data forwarding, the data forwarding is possible andthe data forwarding has been accepted, the NG-RAN allocates, to thecorresponding PDU session, tunnel information for data forwarding overthe NG3 interface; the NG-RAN contains, in the handover requestacknowledgement message, the information indicating that the dataforwarding for a Qos flow has been accepted; for a successfullyestablished Qos flow in a successfully established PDU session, if the5G-RAN has received the downlink data forwarding proposed for thecorresponding E-RAB and the NG-RAN has accepted the data forwarding forthe Qos flow, the NG-RAN allocates, to the corresponding PDU session,tunnel information for data forwarding over the NG3 interface; for asuccessfully established Qos flow, if the 5G-RAN has received thedownlink data forwarding proposed for the corresponding E-RAB, the dataforwarding is possible and NG-RAN has accepted the data forwarding forthe Qos flow, the NG-RAN allocates, to the corresponding PDU session,tunnel information for data forwarding over the NG3 interface; theNG-RAN contains, in the handover request acknowledgement message, theinformation indicating that the data forwarding for a Qos flow has beenaccepted; the tunnel information for data forwarding is specific to eachPDU session; and

a list of PDU session information rejected to be established by theNG-RAN, wherein the list of PDU session information contains the PDUsession identifier and the causes for rejection.

Step 1709: The AMF transmits a PDU session modification request messageto the SMF. If the tunnel information for data forwarding over the NG3interface has been received from the NG-RAN, the AMF requests the SMF tocreate a data forwarding tunnel. The AMF transmits, to the SMF, thetunnel information for data forwarding received from the NG-RAN. Thismessage contains the PDU session to which an EPS bearer belongs. Thismessage can further contain the mapping between an EPS bearer and a Qosflow in the PDU session. This message contains the informationindicating that a Qos flow has been accepted for the data forwarding bythe target NG-RAN.

According to the mapping between EPS bearer and PDU session identifieras well as Qos flow identifier, the SMF knows the information about theEPS bearer that has been accepted for data forwarding.

The SMF allocates tunnel information to an EPS bearer that has beenaccepted for data forwarding, or the SMF requests the UPF to allocatedata forwarding tunnel information to an EPS bearer that has beenaccepted for data forwarding. Or, the SMF allocates tunnel informationto a PDU session that has accepted the data forwarding, or the SMFrequests the UPF to allocate data forwarding tunnel information to a PDUsession that has accepted the data forwarding.

This message contains information about a successfully established Qosflow and/or information about an unsuccessfully established Qos flow.According to the mapping between EPS bearer and PDU session identifieras well as Qos flow identifier, the SMF knows the information about theEPS bearers that have been successfully admitted to handover. Or, thismessage contains the information about a successfully established Qosflow, and the SMF knows the information about an unsuccessfullyestablished Qos flow according to the information about the Qos flow inthe PDU session of the UE and the information about the successfullyestablished Qos flow.

Step 1710: The SMF transmits a PDU session modification response messageto the AMF. This message contains the tunnel information for dataforwarding between the SGW and the anchor UPF allocated by the SMF orthe anchor UPF. This message contains a target-to-source transparenttransmitter.

This message contains an EPS bearer setup list. The SMF obtains,according to the information about a successfully established Qos flow,the list of established EPS bearers which are successfully handed overto the target base station. This message further contains the tunnelinformation allocated to an EPS bearer that has been accepted for dataforwarding, or the tunnel information allocated to a PDU session thathas been accepted for data forwarding.

This message contains Qos flow information to be mapped in the PDUsession in the 5GS. The Qos flow information contains a Qos flowidentifier and/or Qos information corresponding to the Qos flow.

Step 1711: The SMF transmits, through an N4 session establishment or N4session modification process and to the anchor UPF, the tunnelinformation for downlink data forwarding over the NG3 interfaceallocated by the NG-RAN. The SMF allocates tunnel information for dataforwarding between the SGW and the UPF and transmits it to the UPF. Or,the anchor UPF allocates tunnel information used for data forwardingbetween the SGW and the anchor UPF and then transmits the tunnelinformation to the SMF. The N4 session establishment message or N4session modification message contains the PDU session to which an EPSbearer belongs. The N4 session establishment message or N4 sessionmodification message can further contain the mapping between an EPSbearer and a Qos flow in the PDU session.

The SMF transmits, to the UPF, the tunnel information used for dataforwarding between the SGW and the anchor UPF allocated by the SMF orthe UPF. The tunnel information is specific to the EPS bearer or PDUsession that has been accepted for data forwarding.

This message contains information about a successfully established Qosflow and/or information about an unsuccessfully established Qos flow.Or, this message contains the information about a successfullyestablished Qos flow, and the SMF knows the information about anunsuccessfully established Qos flow according to the information aboutthe Qos flow in the PDU session of the UE and the information about thesuccessfully established Qos flow.

The PDU session modification response message in step 1710 can beexecuted before or after the N4 session establishment response messageor N4 session modification response message in step 1711.

There are three methods for performing data forwarding between the SGWand the anchor UPF.

Method 1: The method for data transmission between the SGW and theanchor UPF is that there is one tunnel for each EPS bearer in each PDUsession. The anchor UPF transmits the data of a same PDU sessionreceived from the SGW in a per EPS bearer tunnel to the NG-RAN throughthe tunnel corresponding to the PDU session, that is, the anchor UPFperforms a mapping from multiple tunnels to one tunnel. The anchor UPFneeds to add the Qos flow identifier in a header of the data packet andthen transmits the data packet to the NG-RAN. In this data forwardingmethod, the anchor UPF or the SMF allocates, to each EPS bearerrequiring data forwarding in each PDU session, tunnel information usedfor data forwarding between the SGW and the anchor UPF. For each PDUsession, the number of data forwarding tunnels is equal to the number ofEPS bearers. The anchor UPF knows, according to the information receivedfrom the AMF in steps 1709 and 1711, the number of EPS bearers requiringdata forwarding in each PDU session on the EPS side. The anchor UPF orthe SMF transmits, to the AMF, the tunnel information for dataforwarding allocated to each EPS bearer in the PDU session.

Method 2: The method for data transmission between the SGW and theanchor UPF is that each PDU session corresponds to one tunnel. The SGWtransmits, through a same tunnel and to the anchor UPF, the data of asame PDU session received by a tunnel corresponding to each EPS bearer.In this data forwarding method, the anchor UPF allocates, to each PDUsession, tunnel information used for data forwarding between the SGW andthe anchor UPF.

Method 3: The method for data transmission between the SGW and theanchor UPF is that each PDU session corresponds to one tunnel. The SGWtransmits, through a same tunnel and to the anchor UPF, the data of asame PDU session received by a tunnel corresponding to each E-RAB, andthe Qos and/or flow related information is added on a header of the datapacket. In this data forwarding method, the anchor UPF allocates, toeach PDU session, tunnel information used for data forwarding betweenthe SGW and the anchor UPF. In the response message in steps 1709 and1711, the anchor UPF transmits, through the AMF and to the AMF, the Qosflow information to be mapped in the PDU session in the 5GS. The Qosflow information contains a Qos flow identifier and/or Qos informationcorresponding to the Qos flow. The AMF informs the MME of theinformation by step 1710, and the MME informs the SGW of the informationby step 1713. The SGW can contain the Qos and/or flow relatedinformation in the header of the data packet.

The anchor UPF transmits the allocated data forwarding tunnelinformation to the AMF through the SMF.

Step 1712: The AMF transmits a forwarding relocation response message tothe MME. This message contains a target-to-source transparenttransmitter and an EPS bearer setup list. This message further containsthe tunnel information allocated to an EPS bearer accepted for dataforwarding.

Step 1713: The MME transmits an indirect data forwarding tunnelestablishment request message to the SGW. This message contains thetunnel information used for data forwarding between the SGW and theanchor UPF.

In the third data forwarding method, this message contains the Qos flowinformation to be mapped by the EPS bearer in the PDU session in the 5GSand is transmitted to the SGW. The Qos flow information contains a Qosflow identifier and/or Qos information corresponding to the Qos flow.

The SGW transmits an indirect data forwarding tunnel establishmentresponse message to the MME. This message contains the uplink tunnelinformation for data forwarding over the S1 interface allocated by theSGW.

Step 1714: The MME transmits a handover command message to the E-UTRAN.This message contains a target-to-source transparent transmitter and thetunnel information for data forwarding over the S1 interface. Thismessage contains the handover type. The tunnel information for dataforwarding over the S1 interface is specific to the E-RAB. The presenceof the tunnel information for data forwarding indicates that the targetbase station has accepted the data forwarding. The handover typecontains a handover in the LTE, a handover from the LTE to the NR ormore. The handover from the LTE to the NR can further contain a handoverfrom an LTE base station connected to the EPC to the NR or a handoverfrom an LTE base station connected to the 5GC to the NR. This is becausean LTE eNB may support an interface to the 5GC, or may not support aninterface to the 5GC.

Step 1715: The E-UTRAN transmits a command message for handover fromE-UTRAN to the UE.

This message can further contain the mapping between a Qos flow in thePDU session and an EPS bearer, i.e., the identifier of the EPS bearermapped by a Qos flow and/or the mapped Qos information.

The E-UTRAN forwards data to the SGW. The E-UTRAN forwards data to theSGW on a tunnel corresponding to each E-RAB accepted for dataforwarding.

The SGW forwards data to the anchor UPF. Corresponding to the three dataforwarding methods described in step 1711, the SGW has differentbehaviors.

In the method 1, the SGW forwards data to the anchor UPF on a tunnelcorresponding to each EPS bearer accepted for data forwarding.

In the method 2, the SGW transmits, through a same tunnel and to theanchor UPF, the data of a same PDU session received by the tunnelcorresponding to each EPS bearer. The anchor UPF performs a mapping fromthe PDU session to the Qos flow.

In the method 3, the SGW transmits, through a same tunnel and to theUPF, the data of a same PDU session received by the tunnel correspondingto each E-RAB, and the Qos and/or flow related information is added on aheader of the data packet. The SGW performs a mapping from the PDUsession to the Qos flow. The SGW performs a mapping from the PDU sessionto the Qos flow according to the information received in step 1713.

The anchor UPF forwards data to the NG-RAN. Corresponding to the threedata forwarding methods described in step 1711, the anchor UPF hasdifferent behaviors.

In the method 1, the anchor UPF transmits the data packets of a same PDUsession received from the SGW in a per EPS bearer tunnel to the NG-RANthrough the corresponding PDU session tunnel, that is, the anchor UPFperforms a mapping from multiple tunnels to one tunnel. The anchor UPFforwards data to the NG-RAN according to the session transmission methodin the 5GS, for example, how many Qos flows being used to transmitdownlink data in each PDU session, and how a header of a Qos flow beingset (for example, the header of the data packet contains the Qos flowidentifier or other information). For an unsuccessfully established Qosflow received in step 1711, if the anchor UPF has received the dataforwarded by the SGW, the anchor UPF discards the data. This is becausethe access control in the NG-RAN is performed per Qos flow; however, onthe E-UTRAN side, the data forwarding tunnel corresponds to each E-RAB,and the data forwarding is also performed in the E-RAB level. The levelof a Qos flow is finer than the level of an E-RAB. Since the E-URANcannot distinguish data from different Qos flows or the E-UTRAN does notknow the information about the unsuccessfully established Qos flows, theE-UTRAN may forward the data of an unsuccessfully established Qos flowin the E-RAB to the SGW, and the SGW then transmits the data to theanchor UPF. The UPF discards the data of the unsuccessfully establishedQos flows.

In the method 2, the anchor UPF directly receives data from the SGW bythe tunnel corresponding to each PDU session. The anchor UPF forwardsdata to the NG-RAN according to the session transmission method in the5GS, for example, how many flows being used to transmit downlink data ineach PDU session, and how a header of a flow being set.

In the method 3, the anchor UPF directly receives, from the SGW, thedata to be forwarded in the 5GS. The anchor UPF forwards data to theNG-RAN.

During the PDU session establishment or EPS bearer establishmentprocess, the UE receives, from the network, Qos information and/or Qosflow information of a Qos flow mapped by an EPS bearer. The UEcorrelates the ongoing EPS bearers with the Qos flows in the PDU sessioncontained in the handover command message. For an EPS bearer without thecorresponding Qos flow, the UE can delete this EPS bearer.

Or, the UE obtains, from the handover command message, the mappingbetween a Qos flow in the PDU session and an EPS bearer. The UEcorrelates the ongoing EPS bearers with the Qos flows in the PDU sessioncontained in the handover command message. For an EPS bearer without thecorresponding Qos flow, the UE can delete this EPS bearer.

Step 1716: The UE transmits a handover completion message to the NG-RAN.

Step 1717: The NG-RAN transmits a handover notification message to theAMF. This message contains the tunnel information for downlink datatransmission allocated by the NG-RAN.

Step 1718: The AMF transmits a handover completion message to the SMF.

Step 1719: The SMF transmits an N4 session modification message to theUPF. The UPF transmits the N4 session modification response to the SMF.The AMF transmits the tunnel information for downlink data transmissionallocated by the NG-RAN to the anchor UPF through the SMF.

Step 1720: The SMF transmits a session handover completionacknowledgement message to the AMF.

The session modification response message in step 1720 can be executedbefore or after the N4 session modification response message in step1719.

So far, the process of obtaining information by the handover process inthe EPS in the present invention has been completed, and the dataforwarding during the handover between different systems can be wellsupported. By this method, the handover problem from the EPS to the 5GScan be solved, so that the loss of data is avoided and the continuity ofservices is ensured.

The foregoing descriptions are merely some implementations of thepresent invention. It should be noted that, to a person of ordinaryskill in the art, various improvements and modifications can be madewithout departing from the principle of the present invention, and theseimprovements and modifications shall be regarded as falling into theprotection scope of the present invention.

What is claimed is:
 1. A method performed by an access and mobilitymanagement function (AMF) for supporting an inter-system handover froman evolved packet system (EPS) system to a 5th generation (5G) system,the method comprising: receiving, from a session management function(SMF), a first message including quality of service (QoS) flowinformation; transmitting, to a base station, a handover request messageincluding E-UTRAN radio access bearer (E-RAB) information and the QoSflow information; receiving, from the base station, a handover requestacknowledgement message including information indicating whether a dataforwarding for at least one QoS flow associated with a protocol dataunit (PDU) session is accepted or not, the information being determinedbased on the E-RAB information and the QoS flow information, when thePDU session is accepted by the base station; and transmitting, to theSMF, a second message including tunnel information for the dataforwarding, wherein the E-RAB information includes an E-RAB identifierand data forwarding information.
 2. The method of claim 1, wherein thehandover request acknowledgement message includes information on a listof QoS flows, which have failed to be established, with a cause value.3. The method of claim 1, wherein the handover request message includeshandover type information.
 4. The method of claim 1, wherein thehandover request acknowledgement message includes PDU sessioninformation indicating that the PDU session is rejected to beestablished by the base station and cause information for a rejection incase that the PDU session is not accepted.
 5. A method performed by Auser plane function (UPF) for supporting an inter-system handover froman evolved packet system (EPS) system to a 5th generation (5G) system,the method comprising: receiving, from a session management function(SMF), a message including first tunnel information for data forwardingbetween a serving gateway (SGW) and the UPF, and second tunnelinformation for downlink data forwarding between a base station and theUPF allocated by the base station; receiving, from the SGW, a datapacket through a tunnel for data forwarding based on the first tunnelinformation; and forwarding, to the base station, the data packet in aprotocol data unit (PDU) session tunnel.
 6. The method of claim 5,further comprising: adding a quality of service flow identifier (QFI) onthe data packet through the tunnel for data forwarding based on thefirst tunnel information.
 7. The method of claim 5, wherein the messagefurther includes mapping information associated with quality of service(QoS) flow.
 8. An access and mobility management function (AMF) forsupporting an inter-system handover from an evolved packet system (EPS)system to a 5th generation (5G) system, the AMF comprising: atransceiver; and a controller coupled with the transceiver andconfigured to: receive, from a session management function (SMF), afirst message including quality of service (Qos) flow information;transmit, to a base station, a handover request message includingE-UTRAN radio access bearer (E-RAB) information and the Qos flowinformation; receive, from the base station, a handover requestacknowledgement message including information indicating whether a dataforwarding for at least one Qos flow associated with a protocol dataunit (PDU) session is accepted or not, the information being determinedbased on the E-RAB information and the QoS flow information, when thePDU session is accepted by the base station; and transmit, to the SMF, asecond message including tunnel information for the data forwarding. 9.The AMF of claim 8, wherein the handover request acknowledgement messageincludes information on a list of QoS flows, which have failed to beestablished, with a cause value.
 10. The AMF of claim 8, wherein theE-RAB information includes an E-RAB identifier and data forwardinginformation.
 11. The AMF of claim 8, wherein the handover requestmessage includes handover type information.
 12. The AMF of claim 8,wherein the handover request acknowledgement message includes PDUsession information indicating that the PDU session is rejected to beestablished by the base station and cause information for a rejection incase that the PDU session is not accepted.
 13. A user plane function(UPF) for supporting an inter-system handover from an evolved packetsystem (EPS) system to a 5th generation (5G) system, the UPF comprising:a transceiver; and a controller coupled with the transceiver andconfigured to: receive, from a session management function (SMF), amessage including first tunnel information for data forwarding between aserving gateway (SGW) and the UPF and second tunnel information fordownlink data forwarding between a base station and the UPF allocated bythe base station; receive, from the SGW, a data packet through a tunnelfor data forwarding based on the first tunnel information; and forward,to the base station, the data packet in a protocol data unit (PDU)session tunnel.
 14. The UPF of claim 13, wherein the controller isfurther configured to: add a quality of service flow identifier (QFI) onthe data packet through the tunnel for data forwarding based on thefirst tunnel information.
 15. The UPF of claim 13, wherein the messagefurther includes mapping information associated with quality of service(QoS) flow.